Sagging middles in writing novels can be a mess. Often everything gets thrown into the middle while the plot goes around in circles. New characters or too many characters flounder, back story info dumps create road blocks, tension disappears, dialogue goes on and on by characters who are in their heads instead of taking action, and readers put the book down.
How to fix it? Try thinking of your middle in three parts, beginning, middle, and end. In the beginning of the middle, increase tension, create more conflict. Be sure there are hooks to keep the reader reading. In the middle of the middle, take out info dumps, sub-plots that aren’t needed, minor characters that are distracting and don’t support the plot, and take out dialogue that is unnecessary. The end of the middle should be close to the dark moment. Tension is at a high point, up the stakes for the main character, make the odds seem impossible, include a shock, a surprise, or twist. Check all the events and relationships to be sure the cause and effect chain makes sense and builds to the climax. Above all, avoid aimlessness. Good luck.