Could be the media (unlikely), or the burner (I always suggest Pioneer). Also note that not all games from other regions will work, so that could easily be the problem if you have a NTSC/USA console and are trying to play PAL games. The games that do work - do they work consistently? If they do you at least know the chip is installed okay. If they do not, there is a connection problem. With no discs in the drive, does the Mii Channel and other features work fine? A telltale sign of a loose connection is crashing when entering other applications on the Wii.