I recently purchased a big green egg (large). I have made several dishes on it but I am struggling with the brisket. I bought a whole brisket (about 11 lbs) and set it up with indirect heating on the grate over a plate setter. After about 8 hours, the temperature was 155 degrees. Over the next 10 hours it only rose to 160 degrees on 2 independent thermometers despite a constant dome temperature of 200-225 degrees. The dome thermometer checked out okay. We are near sea level. Why won't the meat get any warmer?