I have had no end of trouble with QI charging. I bought the official Samsung wireless charging back and Samsung QI pad. I had two pads both of which refused to charge and were returned.
I changed it for the Zens pad which worked perfectly - but only for a week before it too packed up.
I don't know whether this technology is usually as flakey as this or whether I have just been unlucky.
Anyway, this Zens pad has now been returned for a replacement