problems with halo 3 skulls

ok

well i know where all the skulls are, and i have collected 2 or 3 today,

then the next time, i go to get the skull, and it isnt there

i was doing it on heroic, which doesnt make any sense

then i did it over again, and i saw it sitting there (sniper scope)

but when i got there , it was gone

waddafuxup?

thanks

http://youtube.com/watch?v=9WrXr4Aa3Bs- This linkgives you the locations in video form of them skulls needed to unlock the achievements.

http://highimpacthalo.org/forum/showthread.php?t=33992- This one gives you a list of every single skull in the game. The achievements ones and the ones that help unlock the hayabusa armor.

i know where they are,

i just cant get them

they dissapear