client refusal to pay, whats my next step?

First, it sounds like a breach of contract (whether written or oral) to which the recipient of the services is claiming that he or she received unsatisfactory service.  The next step is to figure out what the claim is worth.  If the value is high enough, it may be worthwhile to pursue via an attorney.  I would first send a demand for payment, then follow up with a lawsuit to collect the sum due if the demand does not work.  Once again, if the value is low, then it may not be worth pursuing.