I just tried to smoke my first beef brisket yesterday. It was 3 1/2 pounds and according to Jeff's book should have taken 5 hours at 225 degrees with an finished internal temperature of 190 degrees. I used a dual probe digital thermometer which has been check for accuracy. After 5 hours, the internal temperature was only 175 degrees. It took another 2 hours to reach 190 degrees. When I took it out, the brisket was very overdone. Any thoughts on what went wrong?