Scheduled NT Backup

We have a PDC and a BDC on one domain. We used scheduled NT Backup in backing up user files in the BDC. Back-up is run mainly on the PDC.
I tried to run NT Backup manually and I was able to backup all network drives in the BDC that I wish.
When Irun unattended / scheduled back-up, error prompts that there is no permission to backup network drives and folders in the BDC.
How can I backup network drives using unattended / scheduled NT Backup? Is there a way or do I have to include some parameters in the batch file to be run?
Thanks.