I was talking to another writer about that dread syndrome, the sagging middle. This is when your opening chapters bristle with potential and power, but then sags when you get past the set-up into the second act of rising action. We realized that often the middle sags because we've dropped a plot line, or diminished it to a sub-plot that enters scenes only occasionally rather than directing the course of the story.
For example, in an adventure story, the writer might be having so much fun with the action plot, moving the characters from one near-disaster to another, but forget about the internal purpose of the events, to change the main character from a loner to someone capable of affiliating, maybe.
Or-- I see this a lot with romance plots-- the writer concentrates on the romance, showing the conflicts and compatibilities of the couple, letting them conceal and reveal secrets... but forgets about the external plot. For example, maybe the situation that got them together in the first place is an election for mayor. This is the external plot and shouldn't be going on in the background, arising only when the romance flags. Rather every scene ought to involve some problem or solution or event having to do with the election, and the "end of the middle" (the crisis scene) probably ought to be when the election seems lost or the candidate corrupt-- something external that connects the characters to outside events and challenges them in some psychological or emotional way.
Anyway, if you're in the middle of your story, and you're feeling a sag, feeling like you have to drag the reader to the next scene, or that you're replaying unimportant situations (like "eating dinner" or "going on the run") from earlier scenes-- go back to your essential story lines. You might have a central plot, but you probably also have another important plot-- the mystery plot, or the internal journey plot. See if you've plotted that all the way through. I always benefit from outlining the steps involved in the underused plot, like what is involved in the election campaign, or what he has to do to learn to trust again-- break the plot down into steps, and then develop those steps in the scenes I've already planned out or drafted for the non-neglected plot.
And don't forget, the middle is the time of
rising conflict, where the "on-the-brink" situation in the opening
chapters gets more and more intense. This applies to each of the major plots, not just one! If the external conflict is a campaign for
mayor, every scene makes the election outcome less predictable... and the costs
of victory more acute. If the romantic conflict is that the heroine is
disguising her identity, then every scene should bring her closer to discovery,
and her deception should become more dangerous to their growing love.