It might be a failure of the teacher - some concepts are difficult to grasp for some people due to the way their brains think. Some people think in different shapes, different methods. If the method your brain happens to like is the one that is being taught, then you might do reasonably well, by pure chance. If your brain's method didn't fit with the method being taught, you might not learn so well, and thus struggle.

This is evident even with something as simple as counting numbers - see this video http://www.youtube.com/watch?feature=player_detailpage&v=UMk... , and that's just the concept of counting.