I find this happens to me more often than not when I start in a blaze of enthusiasm for a basic premise, image or opening scene and with no real idea where to go after that. So I guess that comes back to the point above about letting my short ideas marinate for a while before cracking the knuckles and firing up Word. Once I've lost the thread for a particular short I find it very hard to pick it up again, unless what I've already got is so strong I'm kind of compelled to just grind it out and see what I end up with.