I had a grand epiphany Friday afternoon about what the underlying trouble with my book was. I was lacking a strong story goal. There's a big-picture goal for the series, but there was never really a moment when the main character said "this is what I want to do now" for this book. And that explained why it was all somewhat lacking in conflict. If there's nothing specific that the characters are trying to do, it's hard to stop them from doing it. Then once I figured out what the goal was (it was already there, just not focused on), I realized that part of the problem is that this goal was being easily achieved all along, so the big, climactic scene in which they achieve their goal is rather anticlimactic.
That means more rewrites, though not of the whole book. There are just a few key scenes that will have to drastically change. They can partially achieve what they're trying to do in those scenes while still failing at this goal, and that will make it more tense when they absolutely have to succeed and have no history of having made it happen before.
There was a moment of whining and moaning when I realized I needed to go back to the start, but then I got over it because there's been something nagging at me about this book not being quite right, and it feels good to have figured it out.
Meanwhile, I've thought of a few new details for that new story idea. I think that's a big part of my motivation for getting my head down and finishing this book so I can write the next one and then write this new one. Well, I said this was going to be a year of focusing on my career.
And the harp, because I have to have some fun (and I'm making good progress).