We’ve all read books with sagging middles. The story starts out with a bang and then seems to … sort of .. meander around a bit. If we are persistent we may find that the story settles down to a strong, bang-up ending.
But with a sagging middle, most readers will never get there.
So, how do you fix it?
It’s a structural problem, like a house with a saggy living room. You need to go to the skeleton of the house and make sure it’s held up properly in all the right places.
A compelling goal. A compelling story nearly always has a protagonist who has a compelling exterior goal. Scarlett O’Hara’s initial goal may seem to be to marry Ashley, but that doesn’t work out, and the war starts, and … what is her goal anyway? Here it is: to hang onto Tara, the family plantation. She’ll do anything for that. And she has plenty of challenges.
Dorothy in Oz has a compelling exterior goal: to return home. “There’s no place like home.” It’s like a mantra for that movie.
And in your book? Does your protagonist really, really, really want something? So badly that if she/he doesn’t get it, it wouldn’t just be disappointing, it would be like dying?
(You can set up interior goals too and nudge your character toward becoming a new person, but that’s not what I’m talking about here to carry the reader through your story.)
All subplots relate to the compelling goal. Some novel-writing teachers call the struggle toward the goal the story spine. Everything has to relate to it, even as everything in the body relates to the backbone. If your subplots are unrelated or even only slightly related, the reader will find them more a distraction than anything else.
Of course, you could have an epic novel with several protagonists, each striving through opposition towards a goal. That has been known to work well, especially if the goals are intertwined. But I suggest you master writing the simpler story before you embark on an epic novel.