Try to do it again... If your next game is not successful either, then there may be some problems. And do it with unlocked teams, just to be safe. And without allied victory, as it has no meaning here. This should solve it.
BTW, what is that achievement again? win against 4 hard AI players? or 5? I don't remember that one, but if it says do it against 5 AI's then maybe theres your problem, if not, then nvm.
I had similar problems with achievements on Steam, but usually it gave them to me after the second try. It seems to ignore some achievements for a weird reason sometimes.
I know it can be frustrating, one of those achievements Steam failed to give me was one of the hardest in Endless Space, as it involved quite some luck to get those circumstances and stupid steam did not give me that, and I have no desire to waste hours to acquire it again.
If everything fails, maybe you could send an error ticket or what the hell to the developers, just be sure you did everything correctly while trying to get the achievement.
