Have cooked dozen of pork butts. Today I'm cooking 2 in my 40" Masterbuilt digital smoker...8.5 lbs each at 225. Normally about a 12 hr process. The one on top is always done first, the one on the lower shelf normally takes 30 to 60 min longer. Today the one on the top shelf hits 190 in 6 hrs. Why? The one on the bottom shelf stalled around185 and continues to cook. Anyone experienced something similar? Any variables that could contribute to this? Thanks.