Stories begin to fizzle. I've read scripts where writers start new stories in the third act because they've run out of story in the second act. Writers can be too truthful. Yes, that's right. They fail to realize that movies are not real life.
Real life is filled with boring passages, repetition and ambiguity.
A good story should seem like real life, and it may embrace the footprints of real experience, but it's shaped by theatricality. Simple reality is not enough, you need a touch more. Writers run short on patience and begin to contrive, often throwing in something sensational or salacious. Another unhappy staple of disaster is the protagonist who goes through Hell but never seems to break an honest sweat. This happens a lot.