It looks almost black enough to throw back in the barrel.
I'll tread lightly here, other members may have comments I'm sure.
From what I have read here and from personal experience what you are finding in your ash pan is the same as everyone else who has used Reading coal.
Reading is into selling reclaimed/refuse coal big time. What you are finding in your ash pan looks like coal but doesn't burn like coal. You can try sifting some of it out of the ashes and placing it back on the fire, chances are it still won't burn.
From eia.doe.gov website:
Refuse Bank: A repository for waste material generated by the coal cleaning process.
Refuse Mine: A surface mine where coal is recovered from previously mined coal. It may also be known as a silt bank, culm bank, refuse bank, slurry dam, or dredge operation.
Some of this coal has been exposed to the elements for many decades, it's "lost it's luster".
Some of the coal wasn't that great to begin with.
Try some Blaschak in bags or consider a trip to coal country to pick up the good deep-mined coal at Superior or UAE Harmony.
Coal quality varies greatly, and will have a huge effect on the performance of your coal burner. If you are buying poor quality coal and only half of it burns you are dumping your hard earned cash on the ash pile!