Hello,
In Synthetic backups, the incremental backups are only suported on a B2D. Refer to the following technote that explains the requirements and procedure for synthetic backup. Ensure that you are meeeting those requirements in order to have successful Synthetic backups:
Title: How to create a synthetic (catalog driven) backup job
http://support.veritas.com/docs/276170
Hope the above helps answer your question. Revert in case of doubts on the same.
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Regards.