For once, I agree with Shaune.
I think it's a simple as this:
The client has a pain. In this case, not enough business. They're in so much pain, they cut a deal with you, not entirely conscious of their future reaction to that deal. (Most people are hopeless at predicting their own reactions to events as yet not experienced - we think we know ourselves, but we don't. Plus we are generally deluded about how kind, ethical and altruistic we are. I once read that if we really "saw" ourselves, most of us would commit suicide, so presumably the delusion is a good evolutionary adaptation to prevent that. Either that, or we try to hold ourselves to higher moral standards than mere mammals are truly capable of).
Anyway, you solve the client's pain. What happens next? They're not in pain anymore, so they don't want to pay your bill. This has happened to me. You give the client the service, and once they're not in pain anymore, they're not bothered if they pay your fees or not. It's not that they're deliberately trying to rip off - it's almost as though they can no longer see the value of what you did, because it has become background noise and part of their everyday situation. In this respect, services are different to products. If I sold you a Mercedes and you didn't pay, you'd have a nagging feeing every time you got in it to drive to work. One of my clients hassles me incessantly when he's in pain (not got enough clients), and then as soon as he's got loads of work, I don't hear from him for months.
You might like to read my article on "Getting Paid":
http://www.attractioneers.com/content/view/25/71/
which may shed some new light on the situation.
Good luck with it!
Jane