I figured out why its not working for everyone and I don't think it has anything to do with your country. If you played the game within the first few weeks of release, 343 sent you an email that contains a code which unlocks every specialization in the game. Everyone that has used that code can use the glitch to get these maps for free, if you haven't then it won't work. Also the email was sent to the account you originally used to setup xbl membership. So for me it was sent to my yahoo even though I never use that email and my account is now tied to my hotmail.
I'm not sure if the glitch still works but I would recommend entering the code and then trying again.
The definitive evidence that video games turn people into mass murderers
