The thing about short stories is that they almost always leave us wanting more. They're a snack, not a meal, but the good ones do leave behind a certain satisfaction--a sort of better-to-have-love-and-lost-rather-than-never-loved-at-all feeling.
Does your story begin where it is supposed to? This is a common problem with rough drafts of novels, but I also see a lot of stories that should have started with the narrative on page two. The writer thinks he needs to take the time to introduce me to his world, or he's so enamored of it he thinks I want the four-hour tour, rather than just slipping in references at appropriate moments. In the narrow slice of life provided by a short story, I think it's most effective to throw your reader right into the crisis. But the crisis action must be plot-provoking, evocative of your world, and show me your character in clear terms. Sounds a lot to ask for, eh? Just know that I hold Electric Spec writers under the same constraints I put on myself.
And to make it tougher, I expect that every paragraph of your action/narrative meet those stringent standards. Think of every scene as sitting on a three-legged stool, the legs being plot, setting, and character. If it doesn't touch on each one your writing isn't tight enough for a short story. (A solid recommendation is once you think you're finished, make a goal to cut your word count by ten percent.) I think some scenes in novels can lean a bit, but a short story reader will topple right out of your story without these three elements propping up each scene.
The best short stories are often more about what you don't say.