Damp coal = OK
Wet Coal = Not OK!!
The manual makes a point of printing this in multiple places, (like the bottom of every page) which i discovered is there for a reason. I too tempted the wet coal gods and was smacked into submission. The only issue I ever had with my stoker was with wet coal right. I fired up the first one i got out in the workshop to learn how to use it, wasn't careful about how wet the coal was and i can't remember if it just quit feeding it or if the fire went out.
The coal I use is usually only damp, sometimes it's wetter in the bottom of the buckets, but I quit pouring it in the hopper at that point and let it sit next to the stove for a day or so. It dries out and then I empty the bucket into the hopper. Never had an issue with this procedure.