Hello. The lower retainer hooks around the lower left back tooth (see pic). This lower left back tooth has a crown on it that has been a problem in that it comes loose frequently. Since the lower retainer hooks around this tooth, when I remove the lower retainer, it pulls the crown off with it. I have used dental cement from CVS to glue in the crown, however it still comes off when I remove the lower retainer. Any suggestions? Thank you.