Sometimes the game accepts the key, says "Activation Successful," but then fails to launch and asks for the key again. After Install, Game Doesn't Start [Fix] - Steam Community
If you can still access the game's files, you should de-authorize your current machine before reinstalling or moving to a new PC to "save" an activation slot. serial number nfs shift 2 unleashed fix
Sometimes the game reports a serial number error simply because it cannot connect to the activation server. Sometimes the game accepts the key, says "Activation