Talk to Keystoker or your buddy and find out how deep the coal-bed is/needs to be as it travels down the grate.
It is no different on a stoker grate than in a hand fired...full combustion happens easier with proper coal bed thickness. Too thin and it will struggle. Is your Koker sitting level? Some have jacked their stoves up some on one end to help thickness. Don't know about your Koker 160, but the A's and bigger have an adjustment for coal bed thickness.
If you change one, either air or feed or thickness, you will need to change the others for best burn.