Am I still liable for an unsigned cashier's check?

I live in California and I sold my car that still had a lein on it. Both me and my buyer went to my bank to handle the paper work. He had a loan-cashier's check which he (my buyer) handed over to the person handling my account. The person handling my account told us where we needed to sign and i released liability and did a bill of sale. About 20 days later the person handling my account from my financial institution calls me and says the check was returned because the buyer didn't endorse the back of the check. Is that my fault or my banks fault?