Security event log, no event 540

In the security event log on my SBS 2003 server, successful
logon/logoff is not getting logged. I have the proper "success"
policies defined for this in the Domain Controller Security settings.
Consequently, my Barracuda 310 Web Filter will not work because it
relies on event 540 to authenticate users for content filtering.

Where would I start to diagnose and repair this? I know by default
the security logging is enabled on SBS 2003 so I don't know why this
one broke.