The soot layer will definitely have some insulating effect on whatever it's covering.
Then I think that is my problem.
With using a little less coal and spreading it it’s no longer fusing together and smothering out. I think the quality of the coal is part of the problem
Finally got around to cleaning the chimney and man did it need it. Decent amount of creasote (always has burned dirty) and I’m assuming a bunch of coal dust. Easy enough to knock down, clean up and back to burning! Leaving the chimney cap off for the remainder of the winter.