I doubt I can be of much help but heres maybe something to start with.....
Can you lay down and look in the ash door and see the linkage where it hooks to the pot? There should be a little jamb nut with an adjustment for your rod, at least thats how mine is and its an older burner. Well, maybe you can unbolt the linkage from the pot and see if you can swivel the fines drop out by hand instead of using the lever??
What I am getting at..... that would at least tell you if your lever or linkage is hanging up or if something is wrong with the actual cleanout on the pot.
Geez..... other than that i dont know what to tell you.
Im sure someone will correct me if I am wrong.... if J.C. or Stokerman sees this maybe they can help you better than I....