Sometimes that happens to me with the heroic. Now, whenever I sign onto Halo, I do not have the heroic map pack and I bet the same principle applies. I figured out how to solve it, so it is not really a pain anymore. I put in the disk and then when I get to the main menu, I click matchmaking. I then switch lobbies to the custom game one, and click on the maps option. It then loads all my custom maps. I then switch back to matchmaking and the maps are there. (it sounds like a pain, but it takes all of 15 seconds)