Never, ever, under any circumstance, throw an idea away. If you took the time to write down even just a paragraph, it clearly has some life or value.
Instead, when you get to the point that an idea cannot be developed further, file it either on a computer or in a cabinet with an appropriate folder.
A week later, revisit the idea and see if anything hits you via inspiration. I have had this happen a few times. I was so spent when originally thinking of the idea that my mind was zapped. A week later, with a clear head, I developed it further.
If your cabinet or folder is growing larger, pick a day to analyze all of these ideas, and either alone or with a friend, attempt to diagnose the stopping point-the point at which the idea halts.
If such a diagnosis can be made, you may have very well solved the problem and may now be able to work past it.
If not, see if these ideas can be used in conjunction. Perhaps the issue isn't with you, but rather that the idea is a storyline, not a total plot. Maybe three of these ideas put together, makes a movie!
Elicit advice from friends or family members who are not in the business. Everyone loves movies and has dreams of being in/making their own, so their advice may spur the creative juices.