Retrospect cannot "upgrade or downgrade" datacompression on the fly. Meaning you must create an entire new backup set to change the datacompression to on or off. I am guessing that when you ran that initial backup the tape drive wasn't set to hardware compression, thus creating a backupset that isn't compressed, and since Retrospect cannot change the compression, it is turing of DC whenever you load. I would attempt creating a new backupset making sure that all hardware compression is turned on and then running a new backup with the new backup set. Hopefully once it has DC on it will continue with the DC in the future.