Tape management in Netbackup

Hi,

I have a problem of Managing Tapes. We use LTO4 Tapes, It can retain 3TB in compressed format;
(1) we use NDMP backups from NetApp directly to tape
(2) I have checked "Allow multiple retention per media"
(3) Still we get 1TB - 3TB data in a single tape and Tape shows Full.
(4) Additionally I get different backup speeds for each NetApp Volume backup, even from the same NetApp filer.

I am finding ways to have a consistent backup but difficult to get due to above problems. Pls help to resolve these problems.