fines would not cause clinkers. fines would restrict the air flow causing more unburned coal and higher coal consumption. Higher heat or poorer coal would cause clinkers.
Higher density coal = more bone or rock = higher ash. More ash, the more it is likely to fuse into a chunk.
When I burned some lousey coal in my AA, I was getting clinkers so big, I though I might have to mix ex-lax with the coal..