The b button does not work. Trust me, I've tried it because it just happened to me a short while ago. I've tried everything to fix it, like unplugging the wires.
Go to the Wii options, then Wii settings, then "Screen" which is under "Calendar". Choose EDTV or HDTV (480P) (NOT Standard TV (480i), which is what you chose when it asked you if you would like to choose progressive mode or standard mode).
Then when you turn on your GameCube game it will then ask you to turn on progressive mode. Make sure you press progressive mode, not standard mode.
Progressive mode does look better on a HDTV.