I do a lot of ribs in my AQ, but have never used a rib rack. From your pic, my uneducated guess is too little heat flow between the rib racks. I'm betting that more time would have produced great results. If it were possible to suspend a thermometer between the racks, I'm thinking the temp during cooking would have been considerably lower than the smoker overall temp. The meat acts as a strong heat sink. Did you check or monitor meat temp in any way?