If the wire or clamp is too low, the valve will not open and the nozzle will click off. If the wire or clamp is too high, the valve will stay open and vapors will escape. Always make sure that the wires and clamps are placed properly. Hoses that are flattened or kinked will restrict the vapor return line. When this happens the vapors cannot return to your underground tank and are therefore released into the air.
Torn hoses allow the vapors to escape out of the tear.
Also, hoses that are kinked, flattened or full of gasoline cause the nozzle to constantly shut off while fueling. This makes customers very unhappy. Check your hoses daily and replace any damaged lines. Sometimes the interior components of the nozzle can fail. When a micro-switch is broken the nozzle will not automatically shut off.