Backup is full instead of incremental

Sbackup is set to make a full backup every 7 days and an incremental backup on intervening days. On those days, while the backup made of my ext4 file system is incremental , the backup of my ntfs data partition is full. This seems to be related to bug 630016 and I have also made a comment there. I suspect it needs a modification to this line in fstab, but I am not clear what.
/dev/sda3 /home/robert/Documents vfat umask=000 0 0