Without having read any of your stories, I'd guess that you are trying to cram too many ideas into one short story.
Write the ideas down on paper and not the stories. Put them away for a bit.
Try some writing exercises. The one below, for example, I've selected from Josip Novakovich's
Fiction Writer's Workshop.
- According to Henry James, a writer wrote a novel from a glimpse of a seminary students' dinner party. Write a scene of a story from a glimpse you have had of a group of people - in a cafe, zoo, train or anywhere. Sketch the characters in their setting and let them interact. Do you find that you know too little? Can you make up enough - or import from other experiences - to fill the empty canvas?
Objective: To find out if you can make much out of little. If you can, great. If you can't now, don't worry, you might later, or you'll have to get your stories from other materials.
Check: Can you visualize these people further? Can you begin to hear one person speak? If not, go back and find a way of talking that might fit one of the people in the group, and carry on from there.
Other ones are:
- In three paragraphs, write about someone and what they are doing. This will help you to improve your observation.
- Write down your three earliest memories and try to make a story from them.
- Describe your home town.
- Write My father never... at the top of a page. Continue the personal bit of writing for about two or three pages. Once done, go through the piece and remove all sentimentality and then remove yourself so that my father becomes Character's father.
- Imagine a strange situation. Rather than write a story, write a plot summary. Look at the plot summary and write about any characters in it. Write about other things.
- Think of an object from your past. Write about it.
These, of course, are just suggestions. If you always write fantasy short stories and these don't suit. Try embellishing them with fantastic aspects (i.e.
my father never told me my late mother was a unicorn 
).
