Hi and welcome to the bituminous forum!
I'm not familiar with your particular stove, so pics of your setup might help as well as details about the coal - where it's from etc., however, some of us here have lots of experience with bit stokers in general.
Ideally you want good stoker coal 1-1/4" max top size x 1/4" (ideally that top size isn't over 1" and is known as "pea stoker" but if the slightly larger size will fit through your screw, you can use it). You want coal that's low in ash (less than 12%) and has a low "coke button" or FSI number, less than 6 is good for a stoker.
For a new stoker setup check a few things - 1. Allow a buildup of loose ash ( you should have at least 6" of loose ash over the tuyeres at all times) - do NOT shovel loose ash, wait for a clinker to form. 2. the effect of air adjustments take a while to demonstrait themselves via the appearance of the fuelbed, so adjust in small increments and wait for about 8 hours to make a determination - this takes patience. 3. Use a barometric draft regulator in your chimney and set it at -.04 on the baro (a manometer is great, but not necessary).