Delta dental insurance said the claim was processed, and a check was sent to my dentist in August. However, my dentist's billing clerk said she did not receive any payment from Delta, she sent me a bill of the full amount plus finance charges in November. What should I do to straighten out this mess?

Ask Delta for proof of payment, they can provide a cancelled check.

But until it is, you owe it, so you pay the dentist to clear up credit issues, ask them for a settlement, by showing them a letter from the insurance, showing they were paid. Get a letter from them stating they never received payment, and send that to the insurance.

Thanks! Fr_Chuck.
After numerous phone calls to Delta and to my dentist's billing clerk, the issue has been resolved. Delta sent my dentist a bulk check, payment for multiple claims; Delta has record of the check number and the check was cashed by my dentist. Instead of crediting my account, it appears to me that the billing clerk credited another patient's account for another claim. She wouldn't admit it was her mistake, but she told me that my dentist had agreed to wipe out the amount.

Way to go, sometimes it takes fighting the system to make it work.