Health Insurance: Clinic Won't Take Secondary Insurance

We applied for pregnancy medicare in June of last year. We never heard anything back until after the baby was born and we already paid the doctor bill. Medicare finally said we were apprvoved for the from and to dates of the pregnancy. I put my wife on my insurance around November because she had no insurance at all. So we called the doctors office to state we were approved for medicare and that we wanted reimbursted. She stated that they do take medicare, but medicare is now listed as my wife's secondary insurance. She then stated they only bill the primary provider? Is this right?

Health Insurance: Clinic Won't Take Secondary Insurance