You will see a temperature drop each time you open the door; the key is to focus on temp rather than time. Each piece of meat has it's own unique characteristics ... for example, brisket and pork butt may stall for hours with one piece or cut but not for another.
The great thing about the Smokin-It designs is that the chamber temp bounces back VERY quickly so the impact on total time is negligible.
Get a good thermometer and measure the temp, not the time.