Do you have the mythic map pack?
Did you download cold storage, the free map?
Sometimes it takes the game a while to load the maps so it will say one or more players do not have the maps. To fix this go into custom games and try to change the map to one of the dlc ones. It will say loading on the screen and after its done go back into matchmaking and the message should be gone. You could also just sit in the playlist and the message should eventually go away once the game has loaded the maps off the hdd.
Finaly you could go to bungie.net and they should have a troubleshooting section that should help you with your problems.
And if none of that works then call 1-800-my-xbox and ask to talk to an agent and tell them your problem. They should be able to fix it.
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