I see two things that can keep your temperature actually low, and one that thing that may make you think the cabinet temperature is low.
Loading the smoker with a large load, will keep the temperature down. The more meat (mass) the more heat will be absorbed, thus keeping the temperature low until the meat temperature begins to rise. Second, the thermometer in the door is not very accurate when there is food in the cabinet. This is especially so if there is meat very close to the end of the temperature probe. Water evaporating from the meat will cool the area around the meat, at the beginning when the meat is just beginning to cook, the difference between the temperature near the surface, and the actual temperature can be as much as 40°F.
A third; that I'm just thinking about could be too much moisture, therefore you will need a wider vent opening. With a large load, that much meat will give off a lot of moisture. Opening the vent to 1/2 to 3/4 open will allow more moisture to escape, and improve your temperature. Trapped moisture in the cabinet will actually keep the temperature down.
The best way to monitor and control the cabinet temperature is to use another type of probe thermometer, and place that probe underneath the bottom tray. Many of us use the remote Maverack ET-73. Try to smoke less amounts of meat, and experiment with your vent opening, to see which setting works best. Rotate your racks so that the meat cooks more evenly.