In his excellent book Dare to Be a Great Writer, Leonard Bishop lists three things that plague the writing of amateurs and which they are reluctant to let go of when they begin rewriting:
1. Dialog that tells content that should be delivered in a scene.
2. Scenes that are overlong and that belabor the point they deliver.
3. Overlong introspections that include polemics.
I'd add a fourth: long passages of indirect speech that should be dialog.
While some exceptionally skilled writers (Wendell Berry comes to mind) can handle long passages that don't include a specific scene of action, most less experienced writers cannot do that without bogging the reader down.
Case in point: I've been reading a manuscript written in first person. The narrator has much to say. Too much. He explains everything overmuch, he philosophizes over much. I'm on page 80 and I don't yet know what the story is about. The author is a good writer in that sentences are well put together, his images are often clever, and his vocabulary impressive. But none of that works without more scenes, more action, more interaction. The overlong introspections and arguments are weighing this reader down.
Whenever possible, create more scenes where more happens. Where the characters' dilemmas are evidenced, where characterizations are revealed (motives, in particular), where people show their feelings, their values, their vices.