XP Pro SP1 Norton Anti-Virus Scheduled Scan not scanning

Our PCs:
XP Pro SP1 and NAV Corp Ed (managed from a server). All users are
instructed to log off (not shutdown) every Wednesday so a scheduled
4am scan can run on the PC.


Our Problem:
Users are saying the scan does not run until they log on the PC the
next morning. We have Win2k PCs also, which scanned at 4am just fine.
So it must be something with the XP PCs.


Does anyone have any suggestions?

Thanks for your time.