You're not alone in this at all. I've been doing web sites for about 9 years and this is one of the greatest frustrations I've had.
The one thing I realized long ago is that many clients become paralyzed when it comes to figuring out what they want to say about themselves or their business. And most clients are not comfortable with writing content.
Unless it's a decent size company with in-house marketing people, it's pretty much a given that the client will be late on providing materials - which in turn means I may not get paid the balance due for quite some time.
This is why I keep a lot of clients in the pipeline at one time. Currently, I have 11 different clients in various stages. Some have disappeared for months, others I only get a response from every couple weeks. It's a LOT of loose ends that drive me nuts. Unfortunately (or perhaps fortunately), I've gotten quite used to it. But there's not a day that goes by that I don't try to figure out a better way.
-Jesse