There are lots of things that can effect its performance;
Check the top vent for obstructions, either animal nests or excess roof material that wasn't trimmed away correctly. (Mine was 50% blocked with rubber roof material!)
While the top vent is off make sure the top fins of the fridge are clean and there's no debris (leaves, nests, etc) sitting on them.
Remove the spiral device from the flue and clean the tube and burner area's of rust.
Check the fridges installation. It should come within 3/4" to 1/2" of the back wall. In back, make sure there's a smooth, clear transition between the top of the fridge and the roof or wall vent.
Vacuum out the entire compartment.
Clean the orifice and burner area with some automotive carburetor cleaner. It will melt all carbon and promote efficiency.