One of the common reasons could be a stale media server entry or Master server not being able to reach one of the media servers.
How many media servers do you have?
Have you decommissioned any media server recently?
Run the following comman...
Take a look at the following technote which explains how Netbackup Selects media for backup.
Hopefully this will answer your question.
Take a look at chapter 11 "Managing Storage Pool" in the PD 6.6 Admin guide: