Backup Files in Vista

On my network domain, I have two computers with extra hard drives.
I backup my users "My Documents" onto these drives using the standard
NTbackup
XP has no problem with this.
Vista will backup to the drive once, but after that it will not.

Permissions and security are domain related - both vista machines are
running professional and the user is signing in with a domain login.

Why wont it automatically backup the files? Why do I get 80070035
error messages?

What is the solution to automatic backups?