During low and slow barbecuing of large meats such as Briskets and Pork Shoulders there comes a time in the process where the temperature of the meat slows down immensely for hours.  This is called the plateau and when you are a beginning to learn how to BBQ this will drive you crazy.  Important thing to remember is not to do too much about it, and leave the door and lid closed as much as possible.  It eventually will get past the plateau and you will see the temperature rapidly increase for a while.  To assist it, you may increase the temperature up 25 degrees or so.  I do most of my barbecuing at 225 and once I hit the plateau I increase the temperature to 250 degrees.  This generally helps get it past the plateau a little quicker.  However, I use an electric smoker so I can just turn a dial.  For example, this weekend I smoked a couple Boston Butts for pulled pork, and on Friday night I checked the temperature before I went to sleep and it was at 160 degrees.  When I woke up Saturday morning, I checked the temperature again and it was at 162 after 8 hours.  I bumped up the temperature to 250 and an hour later I was at 170.

The best things to remember is to trust yourself and equipment and the food will be done when its done, so be patient.