Combines G gleaner grain loss

Concerning your shoe extension -- is the extension to the chaffer adjustable like the front portion or is it made of rods which span the openingIJ

Check the air ducting. Make sure the air has a free flow across the shoe. Sometimes even the smallest abnormality in the duct work will change the air flow enough to overload the shoe in one spot. You can check this by doing a hot kill to the combine, shut it off when it is fully loaded, then check the shoe to see if it has a uniform load across its width.