Why did I not get achievement for completing the mission?

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 slow 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.