Both previous responders are correct, you must have a PROTECTED Full backup in order to do an incremental backup. This Full backup should be done using the same backup specification as the Incremental. I know some people like to have different backup specs for the Full and Incremental, this is not a good idea for this very reason
In addition
- Any change to the backup specification will cause the next backup to be run as Full. I have seen this occur when someone goes into Destination, and changeds the media pool, for example. As soon as they hit the Apply button, the next backup will be full
- Backups must be protected. I would say if you are doing a Full Backup every week, the Incrementals should remain protected for as long as the Full
- Turning off the TreeWalk on Windows systems will cause the backups to be run as Full
- Just to be sure that this wasn't a known problem, I went back through 6 months of Core and Cell Server patches, and found no mention of this
