The two of us should probably form a club, cause I've been dealing with this problem since the dawn of time.
OK, that's blown out of proportion slightly, since I haven't been writing for quite some time between my school days and today, but whenever I'd give it a go, I'd stumble into this problem.
However, I think I've found some answers just these past few days. Not just to that question, but to some others as well. So here's my guess, which I'll form in a question - do you outline? My guess is that you're not, cause many writers who start writing when they get a surge of inspiration (been there, done that, guilty as charged) often fall into the category of non-organisers.
The thing is, while it's great to write from emotions and that wonderful feeling that grabs you in the moment of inspiration, you can't count on that to carry you over the finish line. It might be possible if we're talking short stories, but not in case of any longer works of fiction. One problem arises, and that problem is that you end up without something to guide you once the inspiration is gone. Suddenly, you're not sure where were you going with the story, and don't really feel like continuing with that particular piece of work.
Of course, one of the most notable habit (or a personality trait) of the vast majority of writers is that they act on impulses and live for those periods of creativity, which is why they have shelves and closets full of papers with unfinished stories, left for another surge of inspiration, otherwise known as getting kicked in the rear by your muse.
That is precisely why many people will say that you shouldn't use those inspirational moments for writing - you should use that creative surge for planning. Cause a great plan, full of wonderful ideas and interesting twists and turns, fully developed characters and who knows what else, is what'll take you further down the creative road. It's a great long-term approach. You might want to give it a try.
Now all you have to do is wait for another wave of ideas, or try to cause it yourself by stimulating your own imagination.