That's not enough information, but I would check maxschedsessions
parameter and increase it appropriately if it's limiting the total
amount of sessions. Also, you might want to put a disk pool in front of
your tape pool in order to allow all the data to backup to a pool (not
limited by number of tape drives for concurrent sessions) and then force
a migration from disk to tape pool using hi= and low= parameters in a
script. I'll show you mine:

update stg ptdisk_online_90 hi=0 lo=0

and then later reset the disk back so it will hold some data until
migration is triggered:

update stg ptdisk_online_90 hi=70 lo=0

I run these by administrative scheduler at the appropriate times. I
simply call the scripts.

Hope this helps,

Kevin.