It seems like no new challenges are being posted any more…what gives? This makes a few achievements impossible to get now.
This has been an ongoing issue for a matter of months now and there is numerous threads about it. Try looking in the Halo 4 support sub-forum if you want to read more.
As far as I’m aware,the issue doesn’t seem to really have a reason. I think I remember reading that someone has tweeted someone from 343i and they were possibly looking at it? Can’t really say for sure.