I have a number of mk usb double sockets in use downstairs without issue. However, when we fitted one upstairs, it worked for a while, then the usb stopped charging our phones, or any other device. Thought it was a socket fault, so replaced it with another mk socket, and this one won't work on usb either. Is there something I should be doing with the wiring at the back or is it another dud?
The socket itself works.
The socket itself works.

