Without knowing your exact brand and model of smokers, I'll take a shot in the dark: You have 2 problems.
1.) You are most likely getting your temps from the stock temp gauge, which, like mine is total garbage. Get yourself an inexpensive digital thermo, and use that for a reading. Or, even cheaper yet, go to Wally World and get a dial type oven thermo; they are like 3 bucks.
2.) Most charcoal grates on most horizontals are too low to the bottom and don't allow enough ventilation. Plop a couple rocks or bricks under the grate, and you will have plenty of air flow. Not to mention a place for the ash to buildup in a long smoke.
The only other thing that I can think of, is I have seen some folks install their side fire box without punching the knockout in the smoking chamber. For instance, my smoker is a chargriller pro. You can get this as just a grill; with the small 3" damper on the right side. If you install the firebox, you have to punch out the large football shaped opening or you will never get up to temp.
Let us know what kind of smoker you've got and I'm sure we can get you up to molten temperatures in no time