Are they propane burners or some kind of electric burner? Have you checked to see if the only full on or full off is actually the case? How far do the knobs turn between full and low? If they are say 3/4 turn, what happens if you light a burner on full, and then turn the knob half of it's travel? Does the flame seem the same height as full? Maybe the stove is slow to respond, does the flame drop in size after a minute or two when you turn the knob?