For a policy to execute it must be active, and must also have a valid schedule, at least one client, and at least one directive (like ALL_LOCAL_DRIVES)
If you are using version 6, then you may have hit one of its many bugs: basically if a file system on a client fails to back-up on one occassion, netbackup decides never to re-schedule it in the future. Under MP2 and before, the entire server wasn't scheduled, under MP3 the particular file system doesn't get scheduled.
The are two workarounds for this: 1) manually run the back-up for that client. If it's successful, it'll re-schedule itself correctly for the next automated back-up. 2) delete the PEMPERSIST file.
What happens is that when a back-up runs, a 'flag' is written to the pempersist so NBU knows not to try and run the back-up a second time in parallel. When a back-up fails for some reason, the flag isn't cleared so NBU thinks that that server is always still running a back-up so doesn't try to start another one.