Game Completion Glitch?
I beat the game on hard and neither the Hard as Nails or the Nuck Chorris achievements popped.
I had done a few levels the previous day.
I found out I had to replay those levels in order for the 2 achievements to pop.
So if you're having trouble getting achievements to pop, I would suggest either playing the whole game in one sitting, or replaying the levels that don't have a picture of your character at the bottom.