I think that's just what's happened to mine. I thought I knew Heath and Genevieve intimately, but do I really? Sometimes they behave in ways that don't make sense with the the character profile/emotional conflict I've given them.
At this point, I'm in the middle of the book - a very crucial time - and for it to be believable, I have to be 100% certain about my characters and their motivations. But am I? If not, how do I fix this? Probably, I need to think about them more. When I get busy with other stuff, the story recedes and the intensity fades. That's a problem.
My critique partner - who has a very sharp eye for these things - told me she didn't 'buy' Heath's character in the current chapter. Alarm bells. I think that means going back over the chapter, and for me, running it through my head like a movie and in the process, gaining some distance. What would I think if these characters behaved this way on the screen? It's always easy to spot problems when you're not so up close. And perhaps that's just where I go wrong. I write, become immersed in the scene and sometimes don't stop to ask the vital questions. Would he/she say this? How would this make her/him feel? What does her/his choice do to the story? Where can they go from here?
It's incredibly tricky, and something you can only learn through doing. I'm sure - at least I hope - that someday I'm going to have a lightbulb moment about it. In the meantime, I'll pick up the shovel and keep digging.