This is the syntax I use for the ntbackup to backup network drives:
ntbackup backup m:\ /v /a /hc:on /dScheduledBackupm /ld:\logs\backup\backup.log /e /tape:0
(all one line, of course, and where the drive letter m is mapped to the target folder).
By using the logging switch, you get a lot better information on what happened and that might help you track the problem down.
I've occasionally had some problems with the AT command and since Task Scheduler now installs as part of IE, I use that and it seems more stable.
So you might try running the backup manually and making sure it works and then try to tweak the AT part. If it's just the AT part that won't run, check the service account on the Scheduling service.
regards!
paul