Occasionally some of the sites I have made in Umbraco 4 have pages that are for some reason not in the internal cache. The solution is to run the following command while logged in to Umbraco:
http://www.example.com/umbraco/dialogs/republish.aspx?xml=true
This will republish all pages and put them back into the cache. It can take quite a while if it is a large site.