Tentative Meetings not showing up

I work for a major bank and we are having a problem with one of our PA's,
where she is a delegate for her boss and she gets his meeting requests sent
only to her. The problem is that most of them go into her boss' calendar as
tentative (without her actioning them) without a problem, but the odd one
doesn't.

I can't recreate the problem at all or isolate it to one sender, it just
happens randomly. For example one sender might send 10 recurring meeting
requests for the same thing over 10 weeks and all but 1 of them will show up
as tentative. I just can't think of what is causing this. I've changed her PC
and recreated her Outlook profile.

All users are using Outlook 2003 and the PA is not using a Blackberry. She is
also having the following related issue where she accepts a meeting for him
and then 10 minutes later it vanishes from his calendar.

She is not having any problems with her own calendar, its just her bosses.

Any help would be more than appreciated.