There is a weekly volume pool. But when duplication policy start some tapes status become active but NBU doesn't mount them. Some tapes are mounted 25 time, but some tapes are mounted 250 times. What could be a reason related to this issue?. For example, in weekly volume pool there are 20 tapes but some of them are used but others status is active NBU doesn't use them and The status code 96 appears.
Could the problem related to policy?
By default in NBU media are not allowed to have multiple retention levels. So what it means is that if a backup was written to tape with retention level 3 it will not write to this same media a backup with other retention level. So check with bpmedialist command what are the assigned retention levels of media in that volume pool.
That is what I would have checked.