Open Question: Dental Insurance keeps denying my claim, now I am in collections, what should I do?

I had a root canal performed, which my insurance paid for, however, when I got the crown put in, BCBS denied my claim saying Insufficient Xrays. I paid my portion of the services, but BCBS refused to pay unless sufficient xrays were sent. My dentist office tried five different times but the claim kept getting denied. I contacted BCBS and they said I needed to talk to the dental office. The dental office said they have tried five times and refuse to file the claim anymore and it is up to me to get the claim approved. Now I am being sent to collections for $350. I paid my part, I want them to pay theirs, but I am being blackballed into paying since no one is willing to cooperate. What should I do?