For four years, I worked as one of the editorial staff of the Triangulation speculative fiction anthology, and like most publishers, we had a very high rejection rate: over 96% of everything submitted to us failed to make the cut.
        While there were a variety of reasons why we rejected a story, in some ways the most painful reason was because the story was great -- right up until the ending.  The plot intrigued us, we felt tied to the characters, and yet when we read the ending, we shook our heads and rejected the story.  These were the worst rejections to give, because right up until the last page, we believed we'd found a story worth publishing, only to have it completely fall apart on us.
        The most common reason the ending of a genre fiction story fails is because it does not deliver on its promise.  Every story makes an unspoken promise about where it will end up, based on what the protagonist wants versus the conflicts he must overcome to get it.  The main character is in conflict -- be it with Man, Nature or Himself -- and fights to overcome that conflict.  These conflicts, and how the hero deals with them, cause the reader to expect a certain type of ending. 
        And stories that fail to live up to that promise get rejected.
        One story I remember well dealt with a man trapped alone in an office building subbasement, who struggles to survive and escape.  We watched him each step of the way, wincing as he fought to overcome each obstacle to his survival and then cheering him when he did.  Finally, after 20 pages of increasing struggle, he reached the surface -- only to find the city above him had been destroyed.
        The hero hit rock bottom.  All that effort to overcome each problem trapping him, only to find this?  Breathless, we turned the page ... and the story fell flat on its face.  The hero looked around, shrugged, said "Well, it's hopeless" and killed himself.
        This was an ending that failed completely for us.  The author had given us the powerful story of a common man who, in his struggle to survive, found his inner strength and determination ... and yet now we were asked to believe that after all that, that he would now just give up?  It was an ending that could not help but fail, since the entire story had been devoted to him struggling to survive, and succeeding despite the heavy odds against him.  After such a forceful journey of discovery, a man like this cannot just shrug his shoulders and kill himself, and for one good reason: if he really had been that sort of character, he would have taken his own life as soon as he found himself trapped in that subbasement with no hope of rescue.
        A good ending is one that fits the entire story.  As an example, let's look at the last few paragraphs of the mystery story "Grave Consequences," which appeared in Calliope Issue 126.  The protagonist, private eye Mark Sauer, has been asked by a cop friend to look into why someone would steal an unidentified John Doe's corpse from the Manhattan South morgue. 
        During his investigation, he discovers that Joi Li, a Chinese-American girl whom he detests (and who is a part of a criminal triad in Manhattan's Chinatown) has stolen the unidentified dead man, a fellow Chinese.  A recent government ruling in China dictates that all corpses in Guangdong, the most populous province in China, be cremated to save space, which goes against the traditional Chinese customs of burial and ancestor worship.  So Joi intends to turn the unidentified man's corpse over to the Chinese government, claiming it to be that of her cherished dead uncle, and while the government is cremating the John Doe's corpse, she will smuggle the real uncle's corpse into the ancient family tomb in Guangdong.  She asks Mark to not only keep it a secret, but to lie to the police about what he knows, which would violate this ex-Marine's strong sense of what is right.  He tells her to go to hell and leaves her standing there.
        The climax of the story is Mark discovering who stole the corpse, and why.  The denouement that follows should answer the reader's question of what will Mark do?  Will he tell the police what he knows, or will he violate his own principles and lie to the authorities in both the United States and China?
    
Detective Serrano met me at our usual spot at the lunch counter of Bill's Hamburger Grill, on a miserable gray day, with the rain
beating down against the city.  He sat down
         
    
     
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alongside me and ordered a coffee.  "What have you got?"
    
I sat gazing out the window at Joi standing bare-headed in the pouring rain, waiting to see if I would betray her and her family to the authorities on two different continents.
     
Our eyes met.
     
She waited.     
    
I turned my back on her.  She was just some crazy Chinese girl.
    
Yeah.  That was all she was to me.
     
"Well?" Serrano asked.  "What have you got on the case?"
     
I picked up my coffee cup.  "Nothing," I said, shrugging my shoulders.  "I've got nothing at all to tell you."
     
        The story ends with Mark compromising his principles to do what is right, rather than what is legal, because to do otherwise would not make the world any better.  Most readers, after having been kept wondering right up until the last paragraph what Mark's decision would be, feel that this is the best way the story can end.
        Yet the story could have ended any number of ways, although most of those endings would have failed to live up to the story's promise.  For example: Mark tells the police all he knows and lets the chips fall where they may.  This ending disappoints, but is believable since he stays true to his own sense of what is right, as presented in the story.
        But note: this also means that the stolen John Doe's body will be returned to the morgue for burial in a pauper's mass grave, and that the corpse of Joi's dead uncle will be cremated by the government in China, causing his spirit in the afterlife to be unhappy for all eternity.  He will spread his enmity on Joi and her family, and Joi herself will face criminal charges for stealing a corpse from the morgue.  By staying true to his convictions, Mark causes significant pain to a large number of other people.  Because of that, this ending would not resonate well with most readers.
        Or, perhaps, Mark goes to the triad in Chinatown and demands money to keep his mouth shut.  This ending fails completely, because Mark has been shown to be a man with a strong sense of what is right; such a man doesn't accept bribes to conceal the truth from the police.  This ending fails because it betrays what the reader knows about the protagonist.
        Or maybe Mark gets hit and killed by a taxi while on his way to talk to the cops, solving Joi's problem for her. This ending, which crops up a lot in beginners' stories, fails utterly because coincidence has no place in solving problems in fiction, and all editors know it.
        Or maybe there is no ending: Joi asks Mark to cover up the crime, and he just stares at her -- and the story ends.  (I'll let you know right now that most genre editors will turn the page over in confusion, then start looking for a missing last page.)  This is the Unresolved Ending, in which the reader is never told how the story ends. 
        You see this a lot in literary stories, but very rarely in genre fiction.  Genre fiction prefers a definite, clear ending to a story, whereas literary fiction often uses unresolved (or "thought-provoking") endings.  While you might prefer them, be warned that many editors and readers of genre fiction hate them.  A successful ending gives closure to a story; an unresolved ending gives no closure whatsoever, because the author wants the readers to decide for themselves what happened, and how it ends.
        The problem with this is, readers don't want to decide for themselves what happened.  They are not the storyteller here; YOU are.  They are paying you to tell them a fully-realized story, and want you to end the story for them and tell them how it turns out.  Withhold that information, and they feel cheated, which is why we constantly rejected stories with unresolved endings.
        Remember always that you are a storyteller crafting a tale.  Don't leave it unfinished; wrap it up, all the way, and give your readers an ending that leaves them saying "Yes!"
        When you do that, you have a story that will sell.