When the game first came out there was a problem many people who had anti-tracking software on their computer had where connection to XBL servers was blocked. This issue was resolved by editing my hosts file and from then on achievements unlocked like normal at least new ones that is.
Theres been a problem had since release where many achievements I unlocked prior to this fix. Simple achievements like killing the zealot in tip of the spear, looking into your visor in reach, and more cannot be unlocked. I have already been discussing this issue with support for months with no solution. I’m hoping they can work it out but I made this post to see if other people were still having problems with this.