First thing to check is the thermometer. Drop it in boiling water and make sure it is accurate. The thermo should be kept at grate level.
Most SFB's run too hot at the FB side and too cool at the exhaust side; hence the need for tuning plates, etc. Move the thermo around to test different areas of the smoker. Even the best, dialed-in modified smokers vary from one area of the cook surface to another.
Also, extend the smoke stack down to about an inch or so off the grate (flexible metal dryer vent works just fine). This simple mod retains heat and smoke for better control and should boost temps if that is truly your problem.
Last, you might just need a bigger fire. I start mine up with a combo of coals and firewood, and it jumps to 300-350* pretty quickly, then I tune it back down to cook temps.
Keep playing with the unit, and you'll get the hang of it soon. Cheers!