The very fact that you have lots of unburnt coal in your ashpan points the blame at the coal quality, flat bed stoker stoves burn coal completely, if it is coal and it will burn.
Exactly. Anything that makes it's way down that grate and doesn't burn is not coal. Run some good deep-mined coal that's been properly processed and you'll see the difference.
I also have found that different brands of coal burn at different rates and produce different amounts of heat. I have even had this happen with the same load of loose coal.
The number one factor in the appliance's performance is the quality of the coal, number two is the quality of the chimney and if the unit is drafting at the proper rate.
You can buy the best stove in the world (like a LL) and it will burn terribly with bad coal and an improper chimney.
The dealer tells me that can be expected because there is aprox. 18percent shale in coal.