Yeah... sagging middles are the bane of many a writer's existence.
So why do they happen?
There are a variety of reasons, but I can think of mainly three.
First reason: flow. The middle portion of a story tends to be much longer than either the beginning or the ending sections. So odds are that you'll get a sagging middle, if your pacing is off and you're writing too many scenes where nothing important happens.
Second reason: Your ending is in the wrong place. If it feels like a moment in your story's middle should be the ending, anything after that point will feel boring right up to the ending. Even if the climax is after that point. Sad, but true. For proof, think of the movie Casino Royal with Daniel Craig.
Third reason: Stakes. Your story should be raising the stakes for the characters all the time. All the way to the climax. How sharply or gradually this happens depends on the story and characters, but they do have to be raised. Slack down on the raising of stakes and the story will slacken. Especially around the middle.
If you're lucky, it's number one, where the solution might be as easy as a few deletions. On the other hand, the other two reasons require substantial work, so if your story has a sagging middle, try to check out the pacing first.
If the problem is from reason no 2, you might have to cut everything after the point mentioned above out and write it into a possible sequel. Best case scenario that I can think of would be revising the scenes leading into and out of the moment causing the sagging middle, in a way that means that you can take the moment out.
If stakes are your problem, I suspect that rewrites and revisions will be needed, but to see why, you'll have to come back on R-Day.
Sagging middles aren't impossible to solve, but they take a lot of patience and hard work to fix in edits. Which is why I try to keep the middle boosted right from the start. Failing that, from the rewrite onwards.
Look Out for These:
1) Flow issues.
2) Moments that look like the ending, but that aren't in fact close to it.
3) Stakes not being raised.
What reasons do you find cause sagging middles? How do you solve them?