Cause? Meetings not visible in "plan a meeting"

Hello, i am trying to find what causes this problem to occur.

We have an issue that keeps poping up from time to time where one user tries
to plan a meeting for another and when the meeting creator pulls up a
scheduled in the meeting request for another user it wont show all their
meetings. It will show some but not all. This makes meeting planning in
Outlook a bit reliable.

We know how to fix this. /cleanfreebusy seems to take care of it in most
cases. In other cases making sure office is on 2003 sp3. But the problem
keeps returning and we dont know what that cause is. We have a mix of
Outlook but we have seen this problem between people who have the same
version of Outlook.

What could the cause of this problem be?

us:
AD 2003 native
Exchange 2003 current sp/native
Single Mailbox Server
8 Mailbox Stores
Single Domain

Outlook 2003
Outlook 2002
Outlook 2000