Hey Ridge, welcome to the world of handfired coal stoves. Classic problem. I have a Harman considered by some to be one of the best and this happens to my unit too. The older your coal fire becomes in regards to days, the more ash & clinker build up occurs. You will need to build up the fire very hot
and once there, aggressively shake and or poke to remove excessive ash OR ( don't beat me
) allow the fire to burn out. Clean out & relight. Or live with reduced heating capacity. Did you notice how nice & hot your fresh
coal fire was a few days ago?
OK, just reread your problem. Sounds like it is being starved of air but my note above still occurs to stoves of our types.