What's you min/max and how well insulated is your house? You house may cool off fast, and the system can't catch up fast enough. I have had mine at 99% FR when it's really cold and would stayed there for 2 days!! Ripped thru A LOT of coal, but maintained the temp within 1 degree of setpoint. It may overshoot by a degree or two if it's really cranking and trying to catch up.
Also, do you change your temps at all? Setback? I wouldn't recommend it for more than a couple degrees, if that. It takes too long to get it up to temp and then it gets burning hot and overshoots. I leave mine at 68-69, then set back to 67 when we are not home, kick it back up to 69 around 3pm, and nice and toasty at 4:30 when we get home.
Where is the thermostat located from the heat source vents?
Make sure you feed screw is MAX'd out for max feed rate and let the CoalTrol control it.
Maybe change your HLF (Heat Loss Factor) to 1. That will bump the ramp rate faster. I had to set mine on 1, since our house cools off quickly.
My Hyfire stays within 1 degree most of the time.