I have 19/19 on Supersoldier.. I've had it since a few days after the game came out and Crytek has yet to acknowledge it. I AM NOT THE ONLY ONE HAVING THIS PROBLEM!

http://forums.xbox.com/36104529/ShowPost.aspx
http://www.xbox360achievements.org/forum/showthread.php?t=301680
Video showing that I beat the game on SS, 19/19 with no achievement: http://www.youtube.com/watch?v=WSMpiGAlr7U

All I can tell you to do is to wait and see if this issue was addressed in the forthcoming patch.

Because Crytek thought they'd teach people a valuable lesson about gaming, and that is that you don't buy them for achievements, you buy them to play and have fun.

Nah, I don't know the real reason. Just enjoy the game.

Dude, try playing last level again on SS, see if it works, thats how my friend got the achievement

and I got achievement in one shot

I guess you can try replaying the intro level as well as the last one like Bass mentioned above.

This method has worked for some people but others are stuck in the same rut.

I'm also having this issue. I've replayed the first and the last level. Nothing seems to work.

I'm holding out on my SS playthrough until I read a patch note that says it was fixed.