I'm having the same problem with my Keystoker Koker. I believe the response from Keystoker is to shut down the stove/furnace/boiler and look for areas which cause resistance to the coal's flowing from the hopper to the fire area. Supposedly sanding/smoothing any surface irregularity will allow the coal to flow unimpeded through the stoker mechanism.
I believe I've see more unburned coal when my furnace has more idle time, during warmer temperatures. Seems, with cold weather, with the Koker really cranking out heat, I see fewer bits of rice coal in the bottom of the unit.
I'm currently cleaning up these unburned bits of rice coal with a shop vac. I plan to screen out the ash from the unburned coal; and, put the good coal back in the hopper---so, I'm really not really wasting coal yet, it hasn't been thrown out with the ashes. It's just seems like a waste to see this perfectly good coal ending up at the bottom of the furnace ash collector.
My problem is so small, I don't plan on looking into the problem until the heating season is over.