Backup Exec 2010 R3 - Tapes not overwritable?

Hi,

a customer is using 2 sets of tapes.
1 set is used for 1 week.
The next week the set is changed.

Eventually 1 of the sets will get full. (+- after 4 weeks)
So we have a 3rd set which we then erase, which holds data from 4 week ago.
Then we assign those blank tapes to the same media set.

This has always worked, however not today :(
For some reason the blank media, assigned to the same media set, is not overwritable and thus, the backup fails after 1 tape is full ?

Screenshot:

http://i58.tinypic.com/rvle7t.jpg

I've ran the backup last night, but it was queued because the first tape was full...
As you can see, the other tapes, are set to 'overwritable', and they're in the same media set.

Thanks in advance