Ask just about any writer what the most difficult pages to write are, and I bet you'll frequently hear, "pages 60 through 70." If not that, then "the second half of act two" or "the pages right after the midpoint." Ask me the same question, and I'll invariably tell you the same thing.
I like to call it the "60-70 slump" or, a bit more generally, the "Act Two hump." Basically, these are the pages that just plain suck to write. Act One often breezes by. It involves a lot of set up, but also a lot of fun. The stakes aren't necessarily too high yet - in fact, theoretically they're as low as they'll be throughout the whole script, since the stakes should build steadily as the film goes on. This first half of Act Two (pages 30-60 in a conventional three act, 120 page script) build rapidly, as does the action or conflict or tension. The midpoint (60) hits, and should hit hard - for better or worse. The protagonist meets with great defeat here or is thrown off the path he's been following or experiences a great success from which she might fall later. Things change at the midpoint. It's what comes next that's the hard part.
I often get past the midpoint and, even though I know what has to happen in toward the end of Act Two and into Act Three, I usually stop and think, "what's next?" Things often just stagnate here. My characters and story are not yet at a point where the later events can come to fruition - there are a couple missing beats in between - but they also can't backtrack. In short, they're stuck. Rather, I'm stuck.
With the firefighter script that I'm working on now, I spent no fewer than two days last week essentially staring at a blinking cursor for an hour a day, trying to figure out what would come next and why my characters couldn't just skip ahead to the end. Granted, I needed the pages in between to bulk up the script, but the main concern in the writing for me at this point is the actual story progression. If it seems like the natural next move is the one that immediately brings about the end of Act Two, then perhaps I've missed something earlier or have not given enough credit to something that needs to happen. Of course, it could also mean that my story's short and warrants a bit more exploration somewhere.
Ultimately, in this case, the solution came about in two ways. For one, I realized that the order of a few scenes building up to, including, and immediately following the midpoint turn was wrong. I had laid things out in such a way that, in order to fill a scene gap and convey some necessary information, I was going to have to write a highly redundant scene between two characters no more than a couple pages after their last one. Bad news. What's more, other gratuitous scenes and bits of information were going to be thrown in later, since the best places to achieve those moments had already been taken by earlier, similar beats. Reshuffling the scene order not only saved me wasted space in the form of redundant dialogue and action, but it got me thinking about how the rest of the script would likely play out. And when the juices started flowing, they flooded. As if lightning had struck, I raced to hammer out a very terse beat sheet detailing nine or ten sequences that would carry me through most of the rest of the film. They just all clicked right into place, connecting naturally end to end in what seemed like seamless fashion. There's no guarantee that these sequences will generate perfect pacing or pages, of course, but they at least helped me to get through the 60-70 slump, over the Act Two hump.
Don't worry that you're not cut out for this whole writing thing just because you hit a wall in your writing. It's totally natural for writers to experience that at some point. Just work through the slump - it might take some time, but it'll be so worth it when you do - and you'll be in the home stretch.