This has been a problem for me for a while now. A couple of months ago, I received Halo: Combat Evolved Anniversary for Christmas, and for those of you who have also got the game, it comes with a code for the Anniversary Map Pack for Reach. When I redeemed the code, I tried to get some of the easier achievements, such as be the last man in an infection game or get three headshots in a row. I have done both of these things many times over and I still haven't been awarded the achievements. Just in case I have caused any confusion, I also have the Defiant Map Pack and have all of the achievements that came with it originally. The three that came along with the Anniversary Pack I have yet to get.
Any information as to why this is happening would be greatly appreciated.
[Edited on 02.21.2012 5:59 AM PST]