Several users have reported problems playing cross-region games despite they have the newest version of the Wii System Menu for their region.
On my PAL Wii with 4.1E I can't play Ghostbusters (USA-NTSC version), WarioWare: Smooth Moves (USA-NTSC version) or Smooth Moves (Korea-NTSC version), but a lot of other USA-NTSC version games work just fine.
So I suspect it's a more general problem caused by either DriveKey or technical limitations of the Wii (perhaps related to different versions of the Wii disc drive) in relation to playing certain out-of-region games.
(Cases of games not working are quite difficult to troubleshoot, as there are many variables, which could affect it. I assume your Wii is not softmodded, and that you use unscrubbed games and also that the problems are not caused by a bad burn)