You may want to check your concave bars. I run a F-2 and run the cylinder at 3_8. My bars are only 1 year old. I probably have 20 or so heads that end up in the bin.
Ok unthreshed heads are due to either too slow a speed or too much clearance between cyl bars and concaves - of which you should have 3-4-5-6 or 7. A cylinder that is not level will cause this. Just because the gauge said you have 1_8 " clearance it doesn't mean you have that; usually it is more because of wear. If 1_8" won't clean up white caps add more concaves and make sure they are straight and not dished or worn as with your cyl bars. They just don't wear perfectly as the years scrub by.. Also open up the top screen and close down the bottom screen until you get quite a bit of clean grain in the tailings. This should let the unthreshed heads get rethreshed.
like Tbran said, don't believe gauge. Actually go in a measure to be sure. We went through this problem a few years ago with our F3 corn and Soybean special. Had new concaves but found that one side was off almost 1_4" when we measured.