Cached exchange mode not identical to Exchange

Hi,

We are experiencing problems with several users where appointments are shown
in OWA but not in Outlook.
Recreating the .ost doesn't solve the problem
Most of the affected users also use a WMx device with pushmail. These
devices display the correct items most of the time but even here items may be
missing.
Only good copy is on the server..

Additional info : All affected users seem to come from the same old MSX55
Env : MSX2007 with Outlook 2003 SP2 , WM5/6

I've seen messages from other users about this problem but none of them
resulted in a clear solution.