Dentist is coming after me for a bill I thought was settled months ago

I’ll spare the details, but went to a dentist (1st time there) and I mentioned some work I wanted done and they have me a quote and I paid it that day. Got the work done, great job, but now like 5 months later they are coming back to me asking for another payment because my insurance didn’t cover what they thought they would. Now that’s ok, but it has been close to half a year since the work was done. Is this normal? I feel like the transaction was done and any discrepancies should’ve been taken care of months ago. Do I just pay it, or is someone trying to get one on me?