Why arnt the Halo 3 Skulls arnt in their secret hiding spots?

[Erik]

I went to a few sits to figure out all of the spots they are in and I go do exactly what they tell me to but some skulls like famine, thunderstorm and more are never there. Plz Help!!!

 

[maskedmanic]

you have to start on the very beginning of the level.

also you cant get them on easy. so you have to play on normal, heroic, or legendary.

 

[Wiz of Halo]

Tips for successful skull hunting:

-You must play on a difficulty level of normal or higher.

-You must start at the beginning of the stage the skull is on.

--You must successfully "save and quit" the game to save the skull.

--You save and quit after picking up the skull; not before.

- Some skulls are not achievements; there is no indication of retrieval.

- You may check your skull collection in meta-campaign by pressing "X"(blue).

 

[xXDaBombXx]

You have to start the level from the very beginning.