Some people find the middle of their stories sag or drag. This can happen but remembering the three hints for developing a scene should help. Showing characterization, advancing the plot and providing information the reader needs.
The middle is made up of scenes and the writer must become a juggler. Things to remember are what does each major character want, why do they want these objects, such as love, finding the criminal, magical powers and so forth. How does one person's desires effect the other characters?
Every scene in the middle of the book leads to the moment of decision. Sometimes known as the Black Moment. This is when the hero or heroine is faced with a decision that may win or lose them their objective. But that is part of the End of a story so that will be talked about later.
What I've discovered as the story unfolds, one has to look for a number of things in the middle of a book. Are their scenes that are repeats? By this I mean are their repeats of the kind of scene such as a dozen restaurant scenes. Are there scenes that don't really add to the story? Is the scene told from the right viewpoint? Sometimes changing the focus character of a scene brings a fresh eye to the project.
A very important thing is does the scene lead the reader to turn the page. Is there a question they want to see solved at the end of the scene. This has to do with pacing. I've often found switching to a totally different place or character makes me wonder what will happen next.