Why can't i pick the maps when i am party leader in halo 3?

i need help on how to pick the places when i am party leader.|||If you are in matchmaking you cant chose the map or gametype its always random but if you are in the custom game lobby you highlight the name of the map and press a and the list of maps to switch to appears. if my answer is a bit unclear try searching halo 3 custom game lobby on youtube.com