I have model 2D and have smoked several racks of ribs. Yesterday I smoked six racks and the ribs on the lower shelf were burned/heavily charred on the bottom. Shelf was placed about mid way in the box. Any idea as to what happened here? Have never had this happen before. Unit set at 235 degrees and 7 hours as the ribs were more loin back than baby back. Ended up shutting down at 6 and a half hours. Thanks for comments.