We try play “headhunter” and “infection” on Anniversary maps but they modes never appears. What’s the problem?
Thanks.
PD: because of that problem we can’t unluck two achievements 
I believe Headhunter is now only found in Rumble Pit and Multi Team, but to unlock the achievement, it MUST be played in Rumble Pit.
All the DLC maps are integrated into every playlist in the game, but in order for the maps to have a chance of showing up for voting, EVERYBODY in the game lobby MUST HAVE the DLC (but please note that having a full lobby of DLC owners does not guarantee that a DLC map will show). If just a single person doesn’t have the DLC, then the maps will not show up. The chances of matching up with a full lobby of DLC owners is very slim, since the majority of the remaining Halo Reach population do not own DLC, making it much harder to play on DLC maps.
the DLC maps are extremely rare to get online
Unfortunately, the DLC maps are rare to come across. It only comes across if ALL the players HAVE the DLC, & even then, other maps may be shown on the voting screen instead of the DLC.
My advice would be to look through the forums for groups of people who have the DLC, I’ve been able to get most of the DLC achievements because of this.
I hope this helps you with finding the achievements 