Let me try that...I'll get back...thx

Sorry, I feel like this question has been asked before. Once I get to my character select screen, and hold B, and go back to the main screen. The codes don't work anymore. Likewise, after a match, the codes stop working as well. I do not think it is something with my SD Card, because my computer reads it just fine. It's a SD Card that Nintendo releases...one of the official ones. I have looked under search for a while, and I don't know, maybe I just don't read very well, but I can't find a solution.

Does anyone know how to solve this problem?