I just completed the mission Fire in the sky, why did I not receive the achievement for doing this?
If you lose your connection to battle.net for more than a minute or so, Achievements are automatically disabled, and do not get re-enabled for the duration of the mission, even if you subsequently reconnect.
There is a pop-up on-screen that mentions this, but it's easily missed, especially if you are in the middle of directing a pitched fight.
If you have a save near the end of the mission, you can try reloading it, though there's no in-game way to check whether or not achievements have been disabled for the current map. A more provable solution would be to replay the mission from the start. Even if your internet is fine and dandy, it's not impossible Blizzard has been experiencing some launch-day network hiccups.
Another alternative (that I don't think applies to your case) is that you used some of the single-player cheat codes, which will also permanently disable achievements for the current campaign.
Probably server fault since it is first day of game release and they are under a lot of stress.Give it some time it may still appear on your list since it may just be very s*low server response*.
Or you might have just lost your internet connection at the wrong time.
Some people really had bad luck with this sort of thing link
Personally just a few moments ago when i logged to my SC account,my achievements were not loading at all like I haven't completed any of the...But my progress and other things were fine.
You may have been the victim of a glitch in the achievements server today. Blizzard announced that they are looking into an issue that is disconnecting players, preventing achievements and experience gain.
Forum post here.