I have found that very few clients are "honest" when you ask for feedback while you are engaged on a contract, and once you are no longer doing work for them, will not tell you why they no longer use your services.
My experience has been different: I
have received honest feedback. I always ask for it after finishing an assignment for a client. I ask them 3 specific questions, one of which asks what I've done well, and another what I could have done better. I've had really useful feedback via this.
I suggest if you aren't getting any reasonable results that it indicates a possiblew problem - perhaps you need to consider how you're asking for feedback and where
you could improve your 'getting feedback' process?
OK, they're 'still using me' is one measure, but frankly, it's a pretty blunt instrument, and not good enough. Any company who tried to use that as their single, solitary measure of 'customer perception of satisfaction' for 9001 would be laughed at. And rightly so, IMO.