outlook exchange calendar problem

System: Outlook 03 and 07, Exchange Server 03
Users: User A uses Outlook 03
Users X and Y using outlook 07 (on different PCs)
The exchange server restarts every night.

Situation:
User A creates an appointment in his calendar. Users X and Y can also
read it (exchange 03).

Problem:
This time, only the outlook from User X will get the appointment from
User A. User Y didnt get it at all (it happened that User Y got it on
the next day)

Another time its happening, only that User Y will get it and not User
X.