Since you do not indicate your OS type or the NDMP filer type, or the tape drive types, all we can do is throw out SWAG.
I have had issues with my NDMP drives where if the filer/drives were disconnected for a while, I had to manually stop and start the process on the filer. (cough, cough NetAPP)
fcadmin config - Check output - one will be online - that is the active adapter:
Adapter Type State Status
---------------------------------------------------
0a initiator CONFIGURED. online - THIS IS THE ONE TO RESET!
0b initiator CONFIGURED. offline
sysconfig -t ( see no drives )
storage disable adapter 0a
storage enable adapter 0a
sysconfig -t ( see all the drives )
Now you can scan your drives
I have had other issues with NDMP drives, where I had to manually run the OS Level commands to update them.
Of course, any tape drive first must be visible at the OS level - make sure you can run scan or sgscan and see them
If you can see the drive at the OS level, you can run this
tpconfig -update -drive -type HCART -path DRIVE.PATH -nh NDMP.HOST -asciiname NB.DRIVE.NAME -shared NO -drstatus UP -robot NUMBER -robtype TLD -robdrnum DRIVENUMBER
You will need to replace the uppercase stuff with your values.