Hello,
Does the Full backup taken before the differential one succeed? Are you using the modified time backup method? Refer to the following technote in that case:
Title: Backup using "Differential/Incremental - using modified time" option tbacks up all the data rather than just the changed data.
http://support.veritas.com/docs/277538
Also verify if any third party software is changing the archive bit of the files backed up. Refer to the technote below:
Title: When running a Differential or Incremental backup job, all files are backed up as if a Full Backup was run.
http://support.veritas.com/docs/190786
If the problem persists, recreate the jobs and test the results.
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Regards.