"Conflicts with another appointment" not being displayed

[Mark]

I have a user who does not see the message "Conflicts with another
appointment on your calendar" when viewing a meeting notice which does, in
fact, conflict with an existing meeting on his calendar.

In addition, he's also having issues with other associates scheduling
appointments over top of existing appointments on his calendar. When
scheduling a new meeting, the other associates look at the scheduling tab on
their PC and his calendar looks to be free when, in fact, it actually has
something in the time frame they're scheduling him in.

What could be causing this? He is running Office 2003 11.8307.8221 SP3.
The Exchange Server is also running 2003. He has 5 delegates who have Editor
permissions on his mailbox.

I've already tried the following:

1. Running Outlook using the /CleanFreeBusy switch (this sometimes
temporarily resolves the problem but it always returns)
2. Confirmed that he has the correct Free/Busy settings in Outlook
3. Confirmed that he has created new appointments in his calendar recently
4. Confirmed that associates are not creating these appointments while in
offline mode.
5. Confirmed that he does not have any rules automatically moving meetings
out of his Inbox.
6. Confirmed he has the following add-ins enabled:
a. Delegate Access
b. Deleted Item Recovery
c. Exchange Extension commands
d. Exchange Extension property pages

 

[Mark]

I've also noticed that when scheduling a meeting with this user, when I look
at the user's calendar on the Scheduling tab, it only show certain meetings,
but when I minimize that window and look at the user's calendar using the
Open a Shared Calendar link, I can see all of the appointments.

Why would the Scheduling tab limit the number of meetings visible to a user?
I'm guessing this information is cached somehow but how would I repair this
file or force an update?