1-the story is an idea, not a story.
2-the ending is telegraphed too early.
3-the ending is abrupt and unsatisfying.
4-the story is aimless, wandering, and wordy.
5-the writing is too stiff and impersonal.
6-half the story is all backstory.
7-logic is thrown to the wind so readers don't buy it.
8-the plot is old, overused, and boring.
9-the pacing is screwed--either too fast, not fast enough, no variety in the pacing.
10-the writing is too precious, too pedantic, or riddled with cliche.
That's the top ten. I know we can all think of ten or fifty more. They say the short story is easy to write. It is if you know what you're doing. It's a disaster if you wing it and have never practiced the form. The short story is an age-old part of storytelling and those who are best at it are marvelous. We have to take the writing of short stories serious. I will call it an art form, for it is. It's as artful as a poem or a novel without being either.
When short stories fail, we all lose. Be a Bradbury. A Poe. Be marvelous.