I doubt anything can take a book on a wonderfully wind and crazy journey quite like a secret.
I think we can all point to instances in our own lives when we have done or said something completely out of character, all in order to protect a secret. As a teenager, we might have tried to keep a bad test score or a even a romantic relationship hidden from our parents. We might have wanted to keep certain family secrets hidden from friends or classmates.
Often, to accomplish the secret-hiding, we resorted to lying. The lies got bigger. Harder to keep track of. They spiraled outward. Those closest to us began to see through us. Question our motives. Fight ensued. And then...
Well. However secrets were handled in your own personal life, you could begin to feel the tension in that previous paragraph, couldn't you?
A secret can do the same thing for your novel.
The best part is, the entire book doesn't need to completely revolve around the keeping of a secret. You can wedge a secret into a subplot--and it can help add tension throughout the work. Secrets can also spin far enough out of control that they do begin to bleed into the main storyline--and shape the events of the novel. They change how the main character interacts with those closest to him or her. They can even alienate the main character--with no one to call on, they're all on their own to try to resolve the central conflict of the novel.
The next time your WIP starts to sag in the middle, try infusing it with a secret--it might belong to the main character, OR it might belong to someone close to that character. I guarantee a secret will ramp the tension right up.