Some stoves burn the coal to a fine powder, some leave a lot of crusty chunks of ash, This depends on the coal, how hot the fire, how much air feeding the fire
and sometimes the design of the stove's firebox and grates.
Make yuorself an 'L' shaped poker, from 1/4" or 5/16" steel rod,, an old political sign often has this heavy 'wire' for a frame. bend a ~3" 'L' on one end and
some kind of loop or handle on the other end, or if you want to get fancy, add a wood handle.
. Use this poker to get in through the ashpan door, under the grate and poke up through the grate into the dark, cold spots,, I'm sure you will find a build up of ash blocking the airways. A few pokes usually results in a cascade of ash falling out,
Once you figure out your stove's personality, you well really enjoy burning coal..
Can you post a photo of your stove, grates and doors ??