I would follow up with your insurance company just to make sure that a check has been issued for the 60%. It's possible that it wasn't, which would explain why the dentist office wants you to pay the full amount. If for whatever reason your insurance company did not send the check, or the dentist's office did not receive it, you are legally accountable for the full amount due.
1. If your insurance company did issue a check, they should also be able to confirm whether it has been cashed. If the dentist's office has cashed the check, follow up with your lawyer. If the check has not been cashed, contact the dentist's office, confirm whether they have received the check, and have your insurance company re-issue the check if they have not.
2. If your insurance company has not issued a check, find out why. If you can get them to issue a check, wait a week, and then follow steps 1. If they will not issue a check, I would pay the dentist's office in full, and then deal with the insurance company to get reimbursement for the 60%. They can always make the check payable to you.
This is just my opinion on how to best handle the situation, keep yourself out of trouble, do your besst to get it taken care of, etc. I hope this helps!