from flash to short story

Yes. A couple of times, and both times they were more successful as longer pieces. One sold to a pro market I'd been trying to crack for ages, and the other's still being shopped about.

In the first case, the flash had all the basic story in it and all I had to do was flesh it out - it was the dreamlike language I added that sold this, I think, and the problem with the flash was that it was too compressed, with very sparse language that didn't suit the tone of the story.

In the second, the viewpoint was just too limited and there were some problematic aspects to it. I changed very little about the flash itself, but I supplemented it so that the longer story turned into three related vignettes, focussing on each of the three main characters in the story. Original flash became the middle vignette, and the story is now much less superficial, much less black-and-white in its thinking, much more layered and ambiguous.

So those are two approaches you could take, though no doubt there are others. Flesh out what you have, or add a substantial amount of new material.

Well, I don't do writing exercises, but I have turned a very short short story into a novel that sold.

Oh, and I'm about to do the same thing again in a different genre.