Volumes were probably attached to the set/rotation being executed, and they had not expired yet. This can happen when The "Days Before Media Expires" field on the Backup Set Definition panel is set up for a longer period of time (more days) than the "Next Usage Date" on the Rotation Maintenance panel.
If these days appear to be properly coordinated, then verify that the command RBSRLSEXTP (release expired tapes to scratch pool) is being run daily. Marking rotations and running them out of order may also cause this problem.
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