I'm about 60 pages into a script. Every time I come up with ideas for a scene, the most I get out of it is two pages. What I'm writing on page 60 I thought would be near the end. Should I go back and fill in some earlier scenes, or am I closer to the middle than the end?
I agree with Doug that a story should be as long as it is and unnecessary padding will only weaken the final product. That said, you could look at some basic structural tools if you're struggling to write stories that hit the desired length.
Yes, these are formulas and you should not follow a formula to write your story. However, understanding some of these structural templates CAN help when you're thinking about your own work and how to structure it.
Put it simply. A 120 page script is three acts. Roughly 30-60-30. Act two is really two 30 page acts. You could take a look at some Save the Cat Beat sheets and other methods of story breakdown and see how they apply to movies that you love or scripts that you admire.
Another way of looking at a feature length story is in terms of sequences. Typically each is fifteen minutes in length. So Act I is 2 sequences etc. Each sequence should have its own beginning, middle, end. It's own mini-story within the story, with character conflict and resolution that ideally escalates the tension and pushes forward the overall narrative.
Another way of thinking about it is that when you take a look at various act breakdowns, if you separate them out into the key elements of each act you're left with roughly 45 KEY STORY BEATS. So that's 45 scenes with room for another 10-15 to flesh out the story.
I'm sorry for not providing greater detail on these but I think if you explore organically you'll hopefully come to a better understanding and fight a methodology that's helpful to you.
Note that I don't pre-structure my own writing but I've studied enough that I have some basic guidelines at the back of my mind when I'm writing a script.
Currently, for example, I'm writing a 60 page TV pilot. I've broken this down into a 5 page teaser and roughly 4 12-13 page acts.
Best of luck with your script!
I understand that talking about structure in this numerical way is antithetical to most writers. And it was/is to me also. That said, go read anything by Dickens. Either the longer works or even the individual chapters in the Pickwick Papers. You'll see the same basic structural spine forming the frame for his stories.