Change the drivers used by medium changer to Microsoft drivers named Unknown Medium Changer.
Veritas device driver installs onle tape drivers and NOT medium changer drivers.
Make sure that SCSI is of the changer is less than that of tape drives(ideally it should be SCSI ID 2 for medium changer and 3 for the tape drive).
Power cycle the server and the library.(shut down library, shut server,boot up library, wait for the initialisation, boot up server)