This pertains to an automobile, but I think the troubleshooting would be very similar. You may need your manual for the specs or ask the mechanic. Hope this helps
Start at the injector..with a multimeter check the ohm's on it...if open there's the problem..
Next go to the plug and clean it and then check it...
If you still have a problem get a wiring diagram and see if your wires are open or grounded.
Think of the electric system as a water system. The electricity is the water. Anyplace it's flow is interupted is where you have a problem.
Your possible problems could be, bad injector, open wire, bad connector or worse case, bad computer which I don't think as the other injectors are working ok.
If you don't have a way to check the injector swap it with one you know is good and see what happens. If it's still bad then start from the connector at the injector and work yourway back.