outlook 2000 appointment not showing up

New to Outlook 2000 My company had a problem show up 2 months ago and nobody can figure it out. Calendar has accepted appt. show up as bold in it, but if you look they are not in there. If you know when appt. is and try to scheduled another for thattime it gives a scheduling conflict. So it knows it is there but you can’t see it. New appts. are showing up fine, but do disappear at some point before appointment. Not always though it is random. Found one(1)(clean/free/busy switch)answer on Microsoft website, but didn’t fix it. Use backup exec software to back up exchange server. Thought it was problem but haven’t been able to confirm this. Any help will be appreciated.