Are you talking about the complete calendar, or Free/Busy information in the
scheduling tab when you book a meeting?

If it's Free/Busy, this is normal behaviour because the default is to
publish only two months at a time.  This can be changed at Tools > Options >
(Preferences) > Calendar options > Free/Busy.  The setting only affects the
calendar being viewed, so if I change mine to three months, other people will
be able to see three months of my free/busy but it won't make any difference
to how much I see of everyone else's.

You also need to wait a little while for the new free/busy data to 1) be
published to the server and 2) replicate round if you have more than one
public folder server in your environment.