Retention rules on tape do not work: old backups are not overwritten.
Specifics of product design.
When retention rules are applied to an archive, the backup that should be retained is not deleted from the tape but is marked for deletion. Backups stored on tapes are not deleted until the tape is overwritten.
For example, if you select 1-week retention, and each tape contains 1 backup, then 8 tapes are required for properly executing such a scheme. In the condition that you are making backups every day (7 days a week), you must have +1 tape (8 tapes) to allow 7-day retention to work, because all backups on the first one will be marked for deletion and the tape could be overwritten only after backup on the 8th tape is created. Similarly, to use a 1-month retention rule, you must have 32 tapes.
In other configurations, additional tapes may also be required to accommodate the proper execution of retention rules. Since tape is a serial access device, it is not possible to overwrite only a selected backups file: only the entire tape can be overwritten (when all backups on it are marked for deletion or when the tape is moved to Free or Imported pool manually).
Add additional tapes to the pool or adjust the retention rules.