Are you familiar with the flashing process in general -- have you flashed before? If not, you may not realize that the usb symbol only appears momentarily unless you are following the flashing documentation very closely. Flashing almost always works, once you understand exactly what you need to do, and are set up correctly. The main obstacle would be if your usb connector is damaged for some reason. Then you won't see evidence of a usb connection because there isn't one.