BE2012: New tapes not automatically assigned to scratch media set

I'm having an issue where new tapes in the drive are being put into the media set Keep Data Infinitely - Do Not Allow Overwrite instead of Scratch which means that when the library is full of new tapes (we change a few a week) there's a load of blank media, but none can be used and the jobs all queue.  If I manually move them to Scratch (thanks for the new big one click button Symantec!) then the jobs pick up the media and start running again as expected.

Is there a default option i need to change (Backup Exec 2012 Service pack 2) so new tapes go straight into Scratch media?  I've had a good look around but can't see anything.  My 12.5 and 2010 servers do this by default.

Thanks, David