why certain halo 3 skulls i do not get the achievement?

[Clarence D]

when i pick up a skull it give me the achievement but all the skulls not doing that

 

[brandon c]

the white skulls dont give you an achievment but gold skulls do (the main point is one is white one is gold)

 

[CHIVISTA 4 LIFE]

not all the skulls give u achievements only some. if ur gettiing a skull that is suppose to give u an achievement but its not its because u need to start the mission from the start and it needs to be on medium difficulty.

 

[Keith R]

there r 5 skulls were you do not get the achievement there good to have..anyway the max pots a game can give is 1000 so
you cant get the achievement for everything.