Dimmer switches are variable resistors. When used in a low wattage circuit they become extremely hard to adjust. Try adding a 60W light to each in parallel with the heat mat. This will make adjustments much easier and may be enough additional draw to keep the circuits pumping current! Use a red light that way the room will always have a red light glow and the snakes will not notice it. Be prepared for the electric bill to go up also though!