Check the Last Modified date on the items that aren't being archived as you
think they should.  That's the date Outlook uses to determine whether or not
an item is ready to be AutoArchived, and a simple action such as moving the
item to another folder can change the Last Modified date.  More information
and a list of actions that change the Last Modified date can be found here:

http://support.microsoft.com/default.aspx?scid=kb;en-us;295657