It usually happens when it's most important, like when your agent says, "Yes, I'll look at that." or your editor says, "Take one more look before we chisel it in stone." (That's why it takes so long -- all that chiseling.) I look at the three hundred or so pages stacked on the table before me and think, "I don't want to read this thing again. I'm tired of these people, and they're going to have to stay the way they are."
It's a problem. If you read your work one more time, chances are you'll find things to tweak, maybe even big things. And then where will you be? Days, even weeks behind where you planned to be, where you need to be.
What if the dialogue doesn't ring true? What if some characters just aren't clear enough? What if, what if?
You've got to do it, wade through one more time and fix what you can. What's more, you've got to let it go when you've done that. Make that last read-through and say, "This is it. I'm sending it out as it is."
It's like dropping your kid off at college. You know he's ready and he's not ready. You know you're okay with it, but at the same time you're not. All you can do is relax and enjoy that empty nest for a while before starting on the next project.