Flush the cache on each media server. Don't rely upon a reboot because to flush as it appears the cache is written to disk and may survive a reboot. Don't rely it being updated automagically ..sometimes it just doesn't happen
The manual says
bpclntcmd -clear_host_cache NetBackup caches host name to IP address mappings to minimize DNS lookups. Each NetBackup process typically has its own in-memory cache and all the processes also share a cache stored on the file system. This option removes all the entries in the shared cache on the file system. If host names/IP addresses have recently been updated on DNS or other host lookup services, the NetBackup caches may be out of sync for up to an hour. To ensure that NetBackup is in sync with host name changes, you can do the following: Stop NetBackup, run bpclntcmd -clear_host_cache, then restart
