I have this backup job that last ran as incremental on 12/23/14. On 1/6/15, it ran as a full. Can't figure it out.
Are you using seperate backup specifications for your Full and your Incremental backups? This is a common cause for this issue. You should be using one backup specification, and setting Full or Incremental in theScheduling area.
Also, check the Protection on your Full Backup. Get the session ID of the last full backup, and run
omnidb -session psession_id] -detail Do not include the [ ] characters
Clientname, Mountpoint and Description have to match between Full and Incr to work properly. If you change the Description it will be treated as new Object and trigger a Full-Backup.
Also as Bob suggested, make sure the latest Full-Backup was still fully protected when the Incr was starting.