Setting the combine for the particular conditions you have at hand is a real fussy balancing act between cylinder or rotor speed, concave clearance and wind. Sieve and chaffer settings are important too but what happens before the sieve is more important than some people think. Shutting the sieve down is not necessarily the way to clean up the grain.
If you haven't already done it you could try slowing down the cylinder/rotor and cranking up the concave just a twitch. Do it in small increments. You want to roll the cob over the concave. Sometimes it's just plain impossible to keep from breaking up the cob if it's tough shelling. How fast is the fan running? Shutting down the sieve slows the air to the chaffer so the junk isn't being suspended by the air blast as well and it ends up falling through the chaffer, onto the sieve, over the back of the sieve and through the return. If the corn is light, in the 50-55 pound range or less you aren't going to be able to run enough wind to keep it as clean as you want to. Where are you directing the wind? Is it blowing through the grain as it falls onto the chaffer or is it directed back more and up through the chaffer. Just a few things that you probably already know but this is what came through my mind as I thought about it. Have a safe harvest. Jim