Usually it is necessary to adjust from field to field, or hybrid to hybrid.
If you are getting a lot of fines, you probably need to look at the rear of the concave setting- are cobs coming out whole? If they are split in two the long way, you need to be open more. If concave is where it needs to be, I'd start by lowering the cylinder speed, but making sure you are still getting all the kernels off the cob.
Adjust the sieves as mentioned above. Lowering fan spped will keep the fines in the machine, but you still have fines.
Keep in mind some corns have softer kernels than others. they require slower cylinder speeds to get the job done. It may be difficult to to get some of them to shell perfectly... especially if it is low test weight, or didn't get ripe before frost.