Halo 3 won't unlock achievement?

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I've completed the first mission on HALO 3 (I'm now half way through crows nest) on normal difficulty, but it won't unlock the achievement. I was signed in properly as it gave me other achiebements (fear the pink mist, iron skull, etc) What is wrong?

 

[coolyawadu]

keep playing it over and over hope i hekped or send a report to bungie and they usually level u uyp in halo

 

[joshbliss92]

Exact same thing happened to me! I just kept playing the mission over and over and it eventually came out of whack and gave me the achievement. Good Luck!

 

[dude]

I hate this. but once u play the level a few more times it will give u it