[Plextalk] why would my plextalk turn off

Hi At the weekend, I tried to charge my plextalk pocket from the usb cable plugged in to an usb adapter, when I put my card in the pocket, the pocket kept turning itself off, back on and off again, tried another card in the pocket and that worked ok any ideas please? I think the charging was just coincidence. I don't think the card had a dodgy file on it but there is no way of telling. Regards Adrien