oh I'm sorry, this can happen if the game loses connection with Steam around the time when you get the achievement.

In the current version, the only way to get it again would be to reset all achievements and unlock them again.
If you tell me which achievement it was, I could also try to tell you how to deactivate specifically this one so you can try to unlock it again.

I'll try to fix this in a future update, working with steam achievements is not that simple, I'll hope I'll be able to fix this!