Something is keeping the kill wire from being grounded when you switch off. Could be the wire itself is disconnected or cut, or it could be in the associated wiring around the start-stop switch or solenoid. Following Bruce's diagnosis will probably fix it of course, but if it doesn't, trace the kill circuit and test each section. Sometimes amateur tinkerers working on it previously have done bad things to the safety circuits, and after that you are lucky if anything works.
