Can not change or create appointment
 

I have several users experiencing this problem. They go to their calander
and try to edit or create an appointment and nothing happens. They can view
the appointment but can not change it. They also do not get remiders of
appointments. When they reboot the computer everything works fine for a
while. Some users have this happen several times a day. We are also using
an Exchange 2003 server. Any ideas on how to fix this would be great.