Try restricting the flow through the smoker by cutting off part of the combustion air going to the burning charcoal. If your's has adjustable vents, shut them closed halfway after the charcoal is nice and hot. The vent pipe should have an adjustable cover too. Close it halfway too. That should cause it to burn hotter.
Also, the lid or door on the smoker might fit poorly, allowing a lot of the heat out, and letting cool air in. Check the fit and adjust by slightly bending, if possible.
