Where pacing usually breaks
The opening often stays in orientation mode too long. The reader learns the world, the cast, and the setup, but the story has not fully committed to the central dramatic problem.
The middle often widens instead of tightens. New scenes, subplots, or revelations appear, but the protagonist is not being forced toward harder and more expensive decisions.
The ending often pays off plot logistics without carrying enough emotional aftershock. That can make a novel feel long in the middle and oddly brief at the close.