My cousins are visiting with their Volt. Last time they were here they were able to charge with the OEM cord from my 2012 Prius Plugin. This time when we plugged in using the Clarity ONE cord, there was an amber light instead of the green light that indicates charging. We had to unplug the cord from the wall to make the amber light go away. We unplugged the Volt and later plugged in my Clarity again and all was fine. The green light came on. Why can't the Voltn use the Clarity cord?