Did you have two devices hooked up to the Anker? One of the things I read about them in reviews on their various Amazon offerings was the 27W rating was total. So if you used one of the ports it got the full output, but it was halved if you used both ports. So to get the full 25W potential out of both ports you needed to get a higher total power rating.

edit: I bought RAVpower chargers and they work perfectly.