Sometimes the lock rings are very stubborn. I usually use a brass or bronze punch. This is important because it will not throw off any sparks. A screw driver or steel punch can set the gas on FIRE! Tap the ring counter clockwise and be sure to be aware of any tabs that may need be cleared as the ring rotates. Some WD-40 may help to loosen any corrosion. Spray it on and let is soak for 10 minutes.