Netbackup 7.6 - VTL unable to detect LTO6 Tapes for HP Storeonce 4420

Hi all

Currently running NBU 7.6, created VTL using HP Storeonce 4220 for LTO6.

NBU is able to detect the VTL LTO6 drives and appear under the library side. However when i tried to inventory the library, it return with no media detected.

Is it due to incompability issue for VTL LTO6? if yes, seems like i have to downgrade to the next supportability of the Drive?

Thanks

sngcc