Your chimney has plenty of draft, and it should be able to generate a nice coal fire with enough coal and air. What happens if you open the inlet damper a couple inches?
Give the factory a chance, but if they can't make it perform it is time to break out the wrenches and tell them to come get it. Your oil boiler has less rated capacity and it does the job, so something is not right.
I budgeted for a AHS coal gun boiler for this project I chose this boiler for the fact I could heat with wood also. I have some acreage with fallen oaks all the time and several cords of wood already cut.
My recommendation to those with firewood is to sell the firewood and burn coal at home. Anything made to burn wood AND coal is always a compromise, one way or the other. My wood/coal hand-fed boiler burned anthracite pretty well, but I always struggled with overheating on warm days, big clinkers on cold days, and occasionally a dead fire at 6 am if the boiler had been run hard all night. It has since been replaced with an EFM DF520, and it has been a tremendous upgrade. If I had to do it over again I would have yanked the hand-fed boiler out the same day I closed on the house.
Sorry if I sound like a stoker salesman (I'm not), but I have been down the hand-fired boiler road and I really don't think you will ever be happy with your current setup. There is a reason companies like EFM started out making conversion stokers for hand-fed's. If you can get a full or even partial refund on the hand-fed, I would go stoker shopping. AHS, AA, Keystoker, EFM, Van Wert, etc etc.