a good cleaning can help, however one of two problems may be at the root of this. it is likely that the feed rate of coal is too high for the air; either decrease the feedrate or increase the air. another issue that can cause a lot of headaches is a loose feed belt; you end up adjusting the air for the coal feed, but the belt is slipping, as the stoker runs for long periods such as it would on cold days or nights, the belt warms up and "sticks" better to the pulleys, this causes the feedrate of the coal to increase and now you don't have enough air to burn all the coal being fed, thus the coalfire builds excessively. the fix, of course, is to have a belt that is newer, free from grease and oil, and fits snugly but not excessively tight.
If you have any more problems take some pictures (if possible) of the fire while stoking and exactly how much buildup you're getting.