How to set autolabel for Daily Diff back up

i have a Symantec Netbackup 5.1 Enterprise installed.

Problem: Netbackup keeps overwritting /appending onto the same tape despite the entire list of free tapes/media (label as TAPE01 - TAPE20) i have in my volume pool-->Daily Diff.

my volume pool have > 8 tapes yet netbackup keeps writing on the very first tape (which is TAPE01) on the entire list instead.

Reten period is 1 week

My plan is,I would like to set auto label for Daily Diff back up  .i.e. 1st & 3rd week same tapes and 2nd & 4th week same tapes with 2sets(4+4 medias).

Kindly can any one help