There could be two reason :
- The game is a bad dump, or is corrupted. you'll have to dump it again.
- The game requires a specific IOS base, and you are not using the correct one. Try a different IOS slot (250 instead of 249 for example) in your USBLoader settings.
You can check the game's integrity with WiiBackupManager.
Select the game and choose the "Sha1" option. If it ends green, the ISO is fine, if it's red it's bad.