Did you make sure to lift the heating element up so it is touching the wood box? Don't touch the element while it's hot. Is the wood up towards the front of the wood box where it's hotter?
Did you have something in the smoker to cook? If there's something in the smoker, the heating element will stay on longer to compensate for the lower temperature created by the meat, etc. The heating element will not cycle as much staying on longer. When the smoker is empty, it gets to temperature quickly, and the element cycles more resulting in less temperature by the wood.
Try these and see how it goes.