I tell you, for the first 30 pages I feel like I'm herding turtles into a small hole in a fence. I get on a roll, and suddenly I have 7 turles through the hole and I'm dancing on air. Maybe have a drink to celebrate.
Then, as you say, there are days and maybe weeks where I can't get anything going. I have hundreds of tricks at this point to stoke the fires and help me herd more turtles. These include: journaling my project (a Word document where I whine about the problems, issues, and generally talk about the characters--no one should ever read this garbage, by the way), research topics that come up in the script, read writing books, jump up and down with feathers in my ears...
Sometimes is it works. Sometimes not. When it doesn't, I drink.
And then around the end of the first act (typically around page 30), I don't know what it is exactly, but things suddenly gel, and I'm off and running. Next thing I know, I'm at page 50. Then 80. Then, oh my God, I better watch it, I'm getting close to too many pages.
Happy turtles, easing into the hole. I drink to celebrate.
And then, through some miracle, I get to the end. Stick it in a drawer for a week, then have at the revisions. By far the hardest part of the process (requiring more drinking).
Before you write me off as a horrible drunk, this whole process can take a long time. 6 months. Maybe even a year. So it's not as much drinking as it sounds like. But the important point is, it's a process. Trust it. Don't stress it. Don't force it. Let the turtles go into the holes when they're damn good and ready.
Excited as you may be to get your piece sold, Hollywood can wait. When I worked for New Line, I saw a room filled to the ceiling with spec scripts. Hollywood gets scripts like waves to the shore.
So let your turtles roam free and have fun!