Make sure everything is up to date and reinstall FBI or try using FBI from HBL. If this doesn't work, back up your saves, format your 3DS and SD card, then reinstall everything. And if even THAT doesn't work, then what you need to do next is throw your 3DS in the trash and buy a new one.... jk, there's actually another app to install CIA but I can't recall the name of it at the moment.