2corpses 1 grave, can't get it halo reach?

[bigtone]

I've done the mission where u get the first target locator, I've used it on the 2 wraiths and they both blew up with 1 call, y won't it give me the achievement??? I've done this part ovr and ovr but it won't gimme the achievement, ps I'm on easy difficulty

 

[mr_j526]

The easy difficulty is the reason. If you would read the description for Easy, it says that you can't earn achievements in that mode. You have to be in Normal or higher to get achievements.