Most people do not know how to communicate a story (in any medium, let alone screenwriting).
It is the one defining element that a professional writer can always manage to accomplish that hordes of amateurs can not. And most likely never will.
The majority of screenplays I have read have two problems.
1) Too much story
2) Too little story
It gets even stranger. Most bad scripts have both.
And it is for the SAME reasons that the script has both too little and too much story at the same time.
How can this be?
The key word of telling a story is FOCUS:
What is the story about?
Every scene (or damn near every scene) should relentlessly tell your story.
If your story is about... how the loss of his father effects poor little Johnny, then EVERY scene should reflect that.
- Most pros do this without even thinking about it.
- Most amateurs think they are doing this, but they aren't.
...So instead, they (at least the better ones) throw more plot at it in the hopes that it fixes the problem.
The result is a screenplay that has too few scenes that address the central premise (too little story). But they also have extra material that "fits" logically into the narrative, but has nothing to do with the central premise (too much story).
In both cases, too little and too much story, is a result of the exact same thing: a screenwriter that doesn't know how to tell a story.