Hi BB -
First off, have you tried combining the disparate short story fragments you've got into one? The combining of lots of funky elements can work really well to spur on your imagination. Just boil the ideas down into 1 or 2 sentences, then arbitrarily stick the sentences together in any order and try to come up with a plot that encompasses both.
Second, are you actually coming up with stories, or just scenarios? The first has a quality of movement, i.e. hero or whatever changes or his situation is changed by the story: the second is just an interesting starting-point.
For me, short stories tend to come all at once - beginning, middle and end - and the work that I do thereafter only fleshes out the world and the characters.
There're some good tips by Andy Diggle in the "writer submissions" bit on this site - if you're trying to write for publications I'd say his tips are good no matter what the medium.
Also, the scriptdroids website is worth checking out - particularly the archives. (It's also where you're more likely to get attention from Rennie, Spurrier and other "real" ie published writers, especially if you put up some examples of your shorts that you're having trouble with)
Hope this is of use
James
Link: Scriptdroids