Based on my experience, I'd say your ashes are getting compacted and essentially closing off the air from beneath, not allowing it to come up through the coal fire. Of course you want to shake the fire down regularly, but if you have rock and clinkers in your fire box, which can't get through the grates, they will accumulate on top of the grates, eventually taking up the whole fire box, as well as stifling the combustion air.
If you have a decent fire going, don't be afraid to poke down through the coals and loosen the whole bed. I don't know why some of the guys have such a problem with that and only want to "scratch" from underneath a little bit. Once you get it loosened and establish an airway through the coal, it should burn, barring other problems. If the large nonburnable pieces are accumulating, you'll have to remove them as they accumulate, to maintain your capacity for fresh coal.
Last edited by Dallas
on Thu Nov 13, 2008 12:12 pm, edited 1 time in total.