IWHBYD skull on halo 3 i need help!?!?!?

[Nathan]

i did the sequence (long and short) i killed all the flood started at the beginning of the level was on legendary and saw the rings light up and play the halo theme but no skull why isnt it there its also the only skull i need to unlock the hayabusa armor

 

[Zachary]

I have this skull. My friend did the sequence. Make sure you:
1)start at the beginning of the level
2)Kill all the flood(Optional i think)
3)Do the sequence
4) and the skull should be across the bridge.