Backup Exec 2012 writing to the wrong media set

So BE 2012, all updates installed, on Windows 2008 R2.

I got a tape library and basically 2 media sets for tapes, monthly and weekly used for duplication to tape jobs.

I have tapes from both media sets in the tape library at any given time.

Randomly, sometimes weekly backup duplications jobs goes to the monthly tapes and reassigns them to the weekly media set and vice versa.

What can I do to stop this?

Yes, if for example a monthly job runs and there is no monthly media set tape availble, I would like the job to fail/prompt me to insert a montly tape. Under absolutely no circumstances ever, do I want a backup to go to the wrong media set.