Medal of Honor: Warfighter achievements, according to the game's forum. Specifically, the "Global Warfighters" achievement, awarded for unlocking "a soldier from each unit, and the "Warchief" achievement, awarded for unlocking all soldiers in multiplayer, will not unlock when the requirements are met.
The majority of the complaints seem to center around the Xbox 360 version of the game, though the problem has been reported on the PS3 version as well. For the moment, there doesn't appear to be a solution or workaround. We've contacted EA for comment.