I can summarize a story idea, even provide vivid and very detailed scenes for a story, but write an entire story? Not happening! I'm good when it comes to short stories that are less than 5000 words, but my ideas are so all-encompassing that I know for a fact that it'd take much more than that to get them completely written out into a coherent form.
I definitely understand that, there are times when I realize 'oh crap, my idea is just so BIG, how the hell am I going to write this coherently?!' It's something that I find myself dealing with a lot.
In an effort to unhinge the scattered quality of the above I'm trying some of The Artist's Way techniques and the following:
I surround myself with creative things: art, books, movies, etc. Anything to keep the creative gears greased and moving.
I read books and short stories not just for the story, but Ilook at the way an author puts their work together.
I create an outline to map out a story. You don't necessarily have to go in the order of your outline you can write the parts out of order and then figure out how to transition the scenes. The latter is good for a bunch of scenes you have, that way you can find ways to put it all together into a story.
Or I just write what I can, leave it for a while and come back to it. Sometimes just walking away from something you've been agonizing over can give you a fresh perspective.