I buy from a local guy who picks up the coal from the South Tamaqua Coal Pocket. And this year I'm having the same issue that you are in regards to clinker formation, although I haven't had any issues keeping the boiler going since I plumbed in the primary loop.
I've kept the same settings as last year, but I get huge slag dams forming on the end of the grate. And whiile the stoker will eventually force these into the pan, I keep an eye on them so the fire doesn't end up burning way back on the grate. Because of you I pulled my grate apart and found the same amount of fines loaded underneath, and I thought that would eliminate the slag issue but it has not. The boiler 'seems' more efficient, but not by a measured standard only by my seat of the pants meter.
That said, it's January 11th, I've burned 530.00 worth of coal to keep the house at ~68f since October, so this is merely another learning experience when it comes to burning coal.