The following TechNote indicates that you can get this if none of the files have changed since the last backup.
STATUS CODE: 99 "NDMP backup failure" occurs when backing up a NDMP client.
NetBackup error messages and status codes
http://www.symante...
one more link found
after upgrading a Windows master server to 7.0.1. Image Cleanup completes (Status 0) but is not expiring images.
http://www.symantec.com/business/support/index?page=content&id=TECH138266
