What was the voltage on the 9 with the screen brightness on max when you plugged it in the 12s spot? From many years of boat and ice fishing Lowrance's shut off at around 10.5 volts (volts pulled up on the screen) if the nine shows low age voltage with screen on bright,the 12 might not start. I'd check wire connections and the battery, my big motor will still start fine right before my Lowrance's shut off.