It may be the coal, what type are you burning (supplier, deep mined or strip mined)?
When the stove is on minimum burn how wide is the burning coal band on the grate?
Now that I have answered your question with two questions.
You can try and increase the minimum by a couple numbers and observe. Note that it takes an hour or so to really see the effect of changes so go slow. Also check the unburnt pieces and see if they break, if no that is shale and you probably have strip mined coal which has some amount. I burned Coal from Agel in Scranton last year in my Alaska and there were always what looked like unburnt pieces when I threw out the ashes.
Hope this help a little.