CALENDARS IN OUTLOOK

We are using Exchange 5.5 and Outlook 97 clients to share calendars out.
In Calendar view when you look at the months on the right hand side of the screen the days should appear bold if there is an appt for that day or normal if there isn’t.

What happens though in some instances is that even though there is an appt or all day event scheduled for a day it still appears in normal over on the right hand side.
Vice versa as well so some of them that are in bold don’t have anything scheduled for them on that day!
Very strange and this occurs on the actual local machine calendar as well as other users viewing it over the network.
Is this some sort of Outlook/Exchange bug or know problem? Our Ex Server is at 5.5SP3.