The only thing registration gives you is a certain amount of statutory damages and attorneys' fees.
Here is my experience. When the license is violated its usually a mistake on some ones part. Some one within the client's organization blew it. They will be very reluctant to admit it, so they cover their ass by stonewalling. As a result, a lawyer gets their attention. Hire a lawyer. It works, you get your money. Best $1500 you'll ever spend.