What happens if an order was never signed and made part of official records?

In 2007, I had a hearing in which the judge ruled that all overpayment of child support that I paid be applied to future payments. We were awarded shared parenting-alternating placement. That still would have left a credit but I am not concerned with any reimbursement. My problem is that although I have the letter from the Judicial Assistant from that hearing showing what the judge ordered, there somehow was never a signed order recorded in the court records. Now 9 1/2 years later, child support enforcement is trying to say I owe arrears in which I do not owe. Actually, there was still an overpayment. I have sent them the documentation showing this but they are still pursuing this. What can I do?