i have the same stove, and my first thought is maybe you
have some ash that won't clear out of your last
grate, and the fire can't get any draft back there...
when you shake, (with a good hot fire) are you getting a
nice glow in the ash pan, and a few hot coals coming down?
can you notice the reflection in a clean ashpan if you can
see that glow up under that rear most grate.?
Also, is there any chance that the last grate isn't shaking
with the other 2? I'v not had anything like this happen, but
can imagine something breaking and one grate not moving???