What OS are you running? AIX, Linux, Windows?
Are these on SAN or other connectivity?
Are these IBM tape drives or other? If IBM are you using IBM tape drivers?
What does your error logs, event viewer, journalctl show?
Again, if using ibm tape drivers, what does the atape.<device>.trace files show? (At least on AIX)
If SAN, any errors on the fabric for your ports for both server and tape?
Any adapter errors on your HBA's in your server?
And errors logged in your library?