Try deleting the codes from the code list completely, and then re-entering them. If you are still having problems, you may need to reset your AR hardware. This can be done by going into Options on the main menu and selecting ‘Reset Hardware’. Please note that resetting erases any codes that have been manually inputted onto your system.
NOTE: This isn`t an issue with the very latest version of Action Replay GameCube, which offers all the codes preinstalled and does not offer facilities for inputting your own.