I'm not certain, but could it be that you are only viewing one month of the
calendar at a time?
I'm referring to the feature where you can publish a certain amount of
months of free or busy time. Follow me on this:
Click "Tools"
Choose "Options"
In the box, under Calencar, click the button "Calendar Options"
Then click the button "Free/Busy Options" near the bottom.
Then there is an option to publish __ months of calendar free/busy
information on the server.

Now mine is set to 3 months, and I'm not sure if that means OTHER people can
see only 3 months of my free/busy stuff or if I can view 3 months of stuff on
shared calendars, but maybe this needs to be set on the shared calendar
itself rather than your own?

Not sure this will help, but thought I'd try.
Good luck!