It sounds like you are either in a filtered View or have turned on
Autoarchive.
So first, check which View you are in. I use the Advanced Toolbar so it is
always there on screen in the Curretn View window. Turn on the Advanced
Toolbar this way: View | Toolbars | use both Standard and Advanced.
Check your Autoarchive settings here: Tools | Options | Other | Archive. Is
it on? If so, look at the file location - that's where it's moving old
calendar entries to. You should see that folder in the Navigation Pane of
Outlook.
If this doesn't help - come back and tell us your version of Outlook and the
exact results you got from following these steps.
Regards
Judy Gleeson
MVP Outlook