You often hear writers, myself included, complain that the middle of a book is the most difficult part to write. And plenty of reviewers will point out when a book has a "sagging" middle, when the tension that fuels the story just seems to peter out.
Why is that?
The beginning, aka the set-up, introduces the main characters, the major internal, external and romantic conflicts, the secondary characters and the subplots. You're getting all the balls in the air. The ending is where you resolve everything -- the balls come to rest.
When you're in the middle of the book, you've got all the balls in the air and now you have to juggle them so that none fall down or the audience doesn't get bored watching.
That is not easy.
How do I try to avoid the sagging, boring middle?
Well, I try not to spend too much time on one element to the detriment of another. I try to find the right balance between character development and backstory, subplots, development of the romance, and historical research.
It can be really tempting to put in too much research. How much is too much? Some readers love a wealth of details and information, others do not. I try to find a balance between enough information to keep things interesting and accurate without making the research stand out. I'm not out to impress my readers with the hours I've spent reading about oh, say, medieval kitchens. I'm out to make them feel as if my characters are really in a medieval kitchen.
I have to be careful I don't dwell too much on the subplots and secondary characters, lest I wind up with Subplotus Overwhelmus. I love my secondary characters, so this is something I have to watch. I have to be especially careful if a particular character is, or might be, a potential hero or heroine. This is how one of my secondary characters in The Welshman's Way wound up with a concussion -- I knocked him out to keep him out of the story.
The middle of a romance is where lust becomes love. I have to show the deepening emotional involvement of the characters and the signs that this relationship can last. This is where I also show the increasingly physical intimacy of the characters. Even if it's a marriage of convenience story with a consummation scene near the start, the physical intimacy changes during the middle, to something more tender and more meaningful.
I may have given the impression that a good middle is all about finding the perfect balance, as if it could be solved by a simple mathematical formula. If only it were so easy! Because of course, it's not. With a romance novel, the main focus should be on the relationship between the hero and heroine, so the scales should tip a little more toward that. And some aspects of the plot and the research and subplots and secondary characters may be more interesting and relevant than others. So the scales will tip and dip and sway as you go. To go back to the juggling analogy, sometimes one ball will be up, another down, but it's not as if they're always going in a perfect circle or rhythym.
And that's what makes the middles really tricky. That's why even seasoned writers can stumble and drop a few balls in the middle of a book. And this is also, I think, the part of writing that separates the people who toy with writing a book from the people who can and will write a book.