Then it may not have any feed adjustment, or something might be worn in the stoker bearing area. Hard to say without removing the stoker plates and inspecting them.
The plates just have a big open slot/hole in them and the bearing just runs around and pushing it back and forth. Maybe one of the slider plates that the bearing runs against is bent or something.
Similar to this pusher below, you can see the small plate on the back where the bearing rides against, on some that can be adjusted in/out to make the hole smaller, therefore, increasing the feed rate.http://nepacrossroads.com/download/file ... &mode=view
or just differences in the stokers/gears, etc....if you take the one that is NOT pushing hot coals, you might be able to add a shim or something to make it feed more, then adjust accordlingly the MAX down.