It looks like your combustion air under the grate may be restricted by fines collecting in the plenum. If you let the fire idle down and the unplug the stove and carefully unbolt the combustion fan you can look in with a flash light and see if the plenum under the grate is getting clogged. If I am right you are having to reduce the amount of coal being fed because it is burning slower due to the restriction.
The proper way to clean it is to let the stove go out and vacuum thoroughly under the grate through the combustion fan hole. Also remove the coal and check the grate for obstructions in the air holes (you can twist a drill bit the right size by hand to remove any crusties).
If this isn't the issue and the mounting screws on the stoker are secure, then maybe the rod is turning by itself somehow, a little blue locktite will prevent that. There is no other adjustment I know of.