So... I had four racks of ribs. All were treated exactly the same, same rub at the same time, put in the smoker at the same time. Used the 3-2-1 method. The two racks on the bottom rack of the smoker were nice and tender, the two racks that were right above the lower two were very tough. During the last hour, I switched the ribs on the racks, so that the top two were now on the bottom rack in the smoker. Can anybody help me with where my mistake was?