Active Tapes are sitting idle for two months

Hi,

Environment:

Master & Media Server - Windows 2008 R2
NetBackup Verison      - 7.5.0.4

HP Ultrium 5 Tape Drive - 8

We have created 3 Policy with different Storage Unit with same Volume Pool with Infinity Rentention.

I have 5 - 8  Active MPX tapes in a specific volume pool at a Infinity retention level. Two Active MPX tapes are in half written state that are sitting idle for two months
while other tapes get written to and new tapes are brought into the pool. Even though we have half written active tapes the new tapes are brought in instead of using active tapes.

I freeze all the active MPX tapes except the old Half Wriiten Active MPX tapes. After I trigger the policy it taking the old half written tapes. But why it does not taking the old tapes when all the tapes are in Active even though we have enough space.

Already we have enabled the "Enable Unrestricted Media Sharing for All Media Servers." in Master Server Host Properties.

Maximum numberof partially full media property - 0

Any finding and fine tune may helpful?