outlook 2003 and exchange shared calendar appointments aren't visible

I cannot see the appointments on certain shared calendars (on 2 computers). The calendar opens and the small preview calendar shows bolded dates. If I click to schedule an appointment it says "this conflicts with another appointment" so this makes me think that the calendar is there and functional, even the shared tasks show up. If I schedule an appointment on the shared calendar it shows up on the owner's computer. When I check the sharing permissions on the owners computer all of the various users have "owner" privilages. The shared calendars worked before I did an upgrade from SBS2000 to SBS 2003 R2. It appears to only be certain calendars as I can see another persons shared calendar fine.
Some of the computers are Win2000 and some are XP but the not visible issue occurs on both types of systems