I almost always get to chapter 4, 5, or 6 (or later) in one of my mysteries, and think, "Is this going the way it should? What could I do to add more umph to this book?"
Inevitably, I'll go back and look at it and think, "A ha -- it's got too much character!" You see, I like my characters a lot in my books, and I often spend a chapter (or two or three) working with them, giving them personal problems, making sure they're having angst ... and forgetting about the mystery.
Happened with my latest book. Every chapter had a hook, but it was a personal one, not a plot one. It's got to be both in a mystery -- it's got to combine the personal problems with the characters AND move the plot forward. The plot was moving, but it was inching.
I went back, threw out most of chapters 1-3, and started the book at Chapter 4. It's much, much stronger now, and I have a clear direction for the mystery.
I think from now on this will be my rule of thumb: when in doubt, look back when I hit chapter 5. Is it working? If not: STOP and rethink!