Coal

I was reading a somewhat humorous article written by a former locomotive fireman describing his experience with various grades of coal (though not on the narrow guage). He related an experience of receiving what looked like a well picked load of coal and setting up what seemed to be a textbook fire. Once on the road, he relaxed a bit thinking he had a good fire in the box, only to unexpectedly notice his steam pressure dropping rapidly. When he opened the firebox doors, most of the coal was gone, and there were only islands of fire here and there on the grates.
My question is, what would cause something like that to happen? How do you recover from a situation where most of the fire is gone?
Just curious.