Hi and welcome to the androidforums.
Well, its either a hardware fault which will need the phone returning for repair, or its dirt in the USB socket on your phone.
Using a bright light and magnifying glass plus fine wooden toothpick, look up the usb port for dirt, fluff etc and clean it as best you can if anything is present. Do be very careful as its quite delicate.
If that does not work its probably a hardware fault which you cannot solve yourself.
Hope this helps
Roger