When that side fails to fire check your voltage coming out of the stock headlight plug. Could be a ground issue or something in the headlight switch preventing proper voltage getting to the ballast. Thats where I would start.
Measure the voltage using the connector power and ground wires first, then compare voltage readings with a measurement from a good ground source (ie: battery) to headlight connector positive. You can also read from battery positive terminal to ground wire on the headlight connector. Somewhere you are losing something, preventing the ballast from igniting the filament in the bulb.