I agree, that an 1/8" difference in travel seems to be insignificant. You mentioned worn parts.. and a nylon roller? The roller may be worn, and not providing the full stroke that it once did. But:
You said about 3" of coal, then 2" of fire, then 2" of ash. Is there normally so much coal that is not burning prior to the burning coal?? It could be that the first several inches of the grate has blocked air holes. From fines. You mentioned that you cleaned out under the grate,, did you poke through the holes in the grate with a wire or a small drill bit? If you got a slug of coal with a lot of fines, it could have plugged up the air holes in just a few hours of burning..
Can you get a straightened coal hanger wire through the holes in the non burning section? if not you may have to shut down the fire and clean all the holes and re-vacuum under the grate..