Hey folks, first post. Got an 820 Pro and fired it up with 8 burgers set on 450 degrees after burn in. 30 minutes later, one side was barely cooked. The flame vent was open; what am I doing wrong? The fire was roaring. I did notice a 40 degree difference between the led and temp gauge on the lid (led was higher at 455 vs 400 or so.) any help would be appreciated.