The simple mechanic of long division. I was sick from school for a couple days, when I came back they'd already gone past the instruction on long division and simply expected me to do the problems, which I couldn't because I couldn't make any sense out of them.
It was years later when I was practicing on my own that I that had an insight -- OH you have to carry the remainder and re-divide into it to get the answer. Very simple, but no one bothered to explain that to me. They just couldn't understand what i wasn't understanding.