Mortgage was sold, overpayment is not reflected in balance. Neither company is claiming responsibility. Who do I contact?

Basically I always overpaid my mortgage. However my mortgage was recently sold. I tried to contact my old mortgage company, and they said that the proper balance would be reflected and finalized 5 days ago at the new company and they cannot give a refund for the overpayment. The new company reflects a balance as if I paid the minimum monthly payment, and if I did have an overpayment I need to get a refund from the old company.

Neither company seems to want to help me resolve this. Is there any type of agency I can contact to start getting this resolved or do I need to get into contact with a lawyer?