Let me start off by saying I am somewhat of a Moodle novice still...
I have created several grade categories and when you do this initially you can choose the Aggregation method you wish (i.e. Simple weighted mean of grades, etc.), however if I go in and try to change it the option "Aggregation" is no longer there. I am an admin for Moodle at my college and checked in the Site Admin Grades menu and there is nothing being "forced", so I am not sure why I am not able to change this when I edit.
Any help would certainly be appreciated