overwritable media not overwriting all backup sets
I have a question about the way the OPP works.
We have a backup policy D2D2T using duplicate backup sets to tape. This policy is applied to multiple selections lists per resource/server. So we have a few backup jobs and as a result multiple backup sets are stored on each tape.
My problem is that when a tape's OPP has expired the first job to access the tape overwrites as expected however this might only take up about 30% of the tape. The next few jobs will then append to this media however some of the later jobs will then fail waiting for overwritable media to be inserted.
Some of the older backup sets from the previous fortnight still remain on this tape and are not overwritten even though the OPP has expired for the tape and the date of the old backup set. There would be enough space on the tape for the failing backup job to run had this older backup set been overwritten.
If I erase the tapes before the backup everything runs fine but as this is a manual process obviously I want to avoid this.
Is there any reason or advice anyone can give me on why only some of the older backup sets remining of the tapes are not being overwritten?
My setting are as follows:
Tools>Option>Media Management - Overwrite Recyclable media contained in the targeted media set before overwriting scratch media.
and in each of the tape job options under device and media, Append to media, overwrite if no appendable media is available.
Hopefully I've explained this well, please let me know if I need to expand on anything.
Thanks in advance.