offline files - offline even when inside network

Hi,
a customer of mine is using Vista Business fully patched with latest
updates incl SP1. He is using Offline Files on a Win2k3 SP2 network share.
Even if he is connected inside his home network (ping to server is ok), he
is working on his offline replica. Problem now is that if he is modifying a
file during the working day, the "online file" on the share is not getting
changed - only if he is invoking the sync process manually.

How is Vista detecting that the server is online - and let my user work on
the "online version" of the file?
Eventlog is not showing any special to me.

Thanks
Charlie