Server don't update immediately

I am running a small network with Windows 2003 Server Standard and 6
Workstations 2 Windows Vista and the rest XP. Also the two Vista PC are set
with offline files and syncronizing at logon and off. However, if both the
Vista PC are connected and one creates a file and save it on the mapped drive
it don't update, meaning the other machine cannot see it until it reboots.
What could cause this

Thans in advance.