I've been working on the new novel for some time now, but hadn't been making much progress because something about the overall plot or setting was bugging me.
I finally worked out, around the end of chapter two, that the problem was with the main character--specifically, he wasn't the main character after all. He won't do as a supporting character either, so he has to come out of the story entirely. This, in turn, means the two chapters I've written have to be scrapped because the new main character enters the sequence of events at a different point. As I started restructuring the plot to accommodate this, it began falling to pieces, so basically I'm back to square one on this story.
This is starting to feel like the sort of problem that is best solved by putting it on the back burner for a while, but I'm going to give it a couple more weeks before going to plan B.