Hello Jim, welcome.
When you look at the burning fire in your EFM, is the fire round, symetrical, and is there a ring of ash around the outer ring of the firepot?
If the fire is oddly shaped, with unburnt spots then you probably have clogged air holes in the plates in the firepot. They sometimes get clogged with either rust or coal fines. Do you use the fines-lever every time you change the ashpan?
If you have the fire burning all the way to the edge of the firepot, with hot coals falling off the edge, then some may not be fully burnt..
BUT usually the black pieces in the ash that look like unburnt coal are actually shale in the coal.. they won't burn. To prove this, pick a handfull or two of the black, 'unburnt' pieces out of the ash pan, and put them back on the fire, on one side where you can see them be pushed to the edge of the fire by the fresh coal coming up from below. If it's shale, it will glow red, but when it gets to the edge, ready to fall off into the ashpan, it will be black, and look much like it did when you pulled it from the ashpan.
Did you change coal suppliers ? Some coal sources have more shale and ash, others less.
If the fire is misshapen, you will need to let the fire go out, and empty the fire pot, clean the holes in the tuyers, and if they are loose, warped or missing their pieces of rope gasketing, then they need to be resealed.. you want the combustion air to only have one pathway; the holes into the coal fire.