How to fix sds chuck from becoming loose?

HI I've got a B&D P80-20 drill with sds chuck but every time I have used it on hammer the chuck becomes undone (loosens its grip on the drill bit) the masonary bits have to be re-tightened every 30-60 seconds, this is my second chuck within 3 months, the drill bits become flared at the base which is caused by hitting the adapter for the chuck, I have been told you can adjust the adapter but no-one knows how to, Hope you can help. Chuck is Rohm 15-13L 1/2-20 S2A