Looks like
this. I've cleared the cache and it's still not fixed. Kept a look out on the official website but no word of even having any errors. I've had the missing achievements for well over a year, but just pointing out the issue I had.
Nah mate, I did everything I could possibly think of and it's been the same. I played it on my other gamertag and even that one had errors.
Stage 1 - Purchase MW2's Prestige Edition for $198
Stage 2 - ???
Stage 3 - Profit