Try telling the story (briefly—just a few pages) from the point of view of a secondary character. You won't cover all the ins and outs, but you'll see your characters and the story in a new light. If it turns out that you just don't have enough material or a deep enough conflict for a book, at least you've discovered that before you're one hundred pages in.
Try switching the points of view of your protagonists. Sometimes a scene will take off when you look at the action from the other character's point of view.
Change the setting. Where a book takes place can affect the tone and the energy of a story. It can kill a story or get it moving in a new direction.
Examine your conflict. Is it really enough to sustain an entire book? If not, change it…or simply deepen it. Conflict is key. I've labored over many a book before throwing up my hands and admitting that the conflict simply must be changed for the book to work. After that, the book tends to fly.
Is the backstory too complicated? That can weigh a book down with lots of explanations. Either simplify things a bit or sit down and decide exactly how much of that backstory the reader really needs to know. Decide exactly when and where you'll reveal things (stretch it out) and do so in a variety of ways (internal narrative, dialogue, or after emotionally trying scenes when there may be an impetus for sharing secrets a character might not ordinarily share). Secondary characters can reveal things, but try not to rely too heavily on this method. It steals the story and the emotional impact from your main characters.
Check to make sure that you're "showing" and not "telling." Is the reader living the story through your characters' actions and dialogue or are you spending too much time telling the reader what is happening and what people are feeling? If there's too much telling, your pacing will slow and you may lose your readers. See if you can convey that necessary information in another way.
Check each scene and make sure that every scene has a purpose. If that's not the case, then you may simply be filling pages, and that's dead weight dragging down your story.
If, after all this, the book still won't budge, you may have to admit that this may not be a story that demands to be told. But please don't be discouraged. There are many other stories waiting in the wings.
What about you? Any favorite ideas on how you jump-start a story that isn't singing?
Image above by Ciccio Pizzettaro