I did not say they ground corn. I said they needed to be crowded to be kept full to shell some corn. I have seen varieties that where they would leave corn on the cob but you could not set the concave any tighter or they would split the cob. Solution was to go faster and crowd the corn through the concave.
A JD 9000 series is a lot harder to get used to setting than the 20 and 00 series combines. You have to do it step at a time or you will never get them set right.