why can't i get any skulls on halo 3?

[Jesus]

i play on ledgendary and still can't get the skulls

 

[tdw_rules_u]

first you have to start the level from the begining i have all the skulls if you need help ill add you on xbox live tommarow my gamer tag is SHORTY ASSASS1N. Btw if u have the mythic map packs i can get you those to its really easy but i dont knw were the one on cortana is sry.