Editing and revising your work is par for the course; unless you're some sort of literary genius who can whip off a perfect manuscript with a first draft, you're gonna spend a great deal of time fine-tuning, cutting, lengthening, etc. But just recently I had to completely re-write two chapters in my novel from scratch.
The reason: no action!
My novel isn't a mystery exactly, but there are some mystery elements, in that the narrator, Drake, has to "dig up" some important info to get to the next stage of the story. While writing it, I really just wanted to plow through and get where it needed to be, so what I did in one of the chapters (chapter two, to be precise) was have Drake overhear some vital information (that is, stuff my reader needed to know) on the television news. On the news! Very lazy of me. My friends in writer's group, Christian and Janine, called me out on it, and they were right.
So, chapter tossed. In the re-write, I have Drake getting the info by visiting a drug den, finding a stoolie of his, and intimidating the information out of the guy. Much better, yeah? That way, it's not just an info dump, it's an actual scene, with actual characters.
The other chapter: Drake learns some details about a shadowy secret society that could be behind all his misery. What did I do in the first version? I, um... I had him go to the library and look up things on the internet!!
Good god, man, what was I thinking? Fortunately, I caught myself out on that one. One laborious re-write later, Drake takes a trip to visit a paranoid drunken writer who knows all about the society. He gets the info from him.
The lesson: when you're writing a chapter in which some revelation occurs, try to include other characters as the source of the revelation. Don't hand the revelation to your protagonist on a silver platter. Make him work for it. Use it as an opportunity to introduce new and interesting inter-actions that reflect more than just the new information.
When you need to tell the reader something vital, why not say it with action?