When I'm coming up with a new story, and it just isn't coming out right, there are a couple of things I might do:
1. If what I'm writing is terrible, I might just write out a basic plot anyway, just so I can have it down on paper--or, more accurately, on computer--and won't forget it. Then I can come back to it later when the writing is coming more easily.
2. Sometimes when I'm lacking the words or inspiration to write a certain story, I go back to the works that helped inspire the story in the first place--typically books, because they help get my own literary juices flowing, but also movies or music or paintings.
3. Sometimes I come up with an idea, and it sounds great in my head, but then, when I try writing it out, it becomes clear that it's not such a great idea, I'm not really very interested in it, it's not up my alley, etc. And I just kind of give up on it. That's a terrible thing to do, probably, but there are just some stories that, realistically, you aren't meant to write.