Returned Security Deposit Check got lost per Tenant

[Winn Vu]

I sent her the check for returned security deposit by certified mail to a P.O. box she provided me. The tracking says it was delivered 10 days ago. Today she contacted me and said she did not receive the check. I advised her the tracking number. She said she will work with the Post Office to track it down but asked me if the check has been cashed and who signed it because it was stolen. Since she lied to me many times in the past, I don’t believe it. Who should be responsible? Thank you for any ideas you may have.