Outlook 2003 Calendar Items dropping off calendar

I have had several complaints from users that calendar items are dropping off calendars. This can be single instance or recurring meeting and even had some around appointments. We are running Exchange 2003 SP2 and Outlook 2003 SP2. Many of these problem revolve around delegates/managers. We also have Blackberry in our environment. I've been all through the basic questions on the meeting, but from the user perspective they see this as not their problem. We have followed the best practices provided by Microsoft and they keep telling me they know they haven't deleted the meeting or done anything to cause this problem. I've been search the internet and have had several hits on this issue, but really no resolution. It's very difficult to troubleshoot. Does anyone know of any issues out there...or better ways to troubleshoot these type issues...