Gearbox/trigger is off spec.
Disassemble the gearbox so only the trigger components and the shuttle are installed. Pull the trigger back so that the shuttle will engage the trigger contacts and then use a screw driver to pull the shuttle towards the top of the gearbox. Release the trigger but watch if the shuttle gets caught with the top of the trigger. If it doesn't you can file off a bit from the trigger or the gearbox so that the trigger will return to the original position more forward.
Similar to the above example. I think it's called "trolley float"