KitchenAid grain mill adjustment?

I have a KitchenAid grain grinder attachment that seems like it's coming out of adjustment.  The finest I can grind is about cornmeal consistence which ain't very good.  I've taken it apart a few times and cleaned out any junk.  When I put it together and use my mechanical brain to assess it, I don't see any way to get it to grind finer.  On the finest setting it has about a millimeter of gap between the grinding surfaces.  I don't see any adjustment (other than the big obvious knob on the front I've already played with).  I can't really believe that the grinding disks are wore down that much since it hasn't gotten that much use (maybe 10 gallons of wheat has gone thru it in the last decade).  

I wondered if a donut shaped bushing might have gone missing but if so, it would be on the wrong side of the assembly (making it more open, not more snug) so I doubt that's it.  Here are pics of the inside, the first is with the sloppy grinding shaft pushed towards the mixer to make the gap tight/fine, second is pushing it the other way showing how sloppy it is (this is the grind I'm getting on the Fine setting) and the third is with the grind knob opened up to a coarse grind.

Any ideas?