Sometimes it is just that the teacher's style of learning doesn't match the students. A teacher who thinks in pictures, for example, may not be able to teach a student who thinks in words.

I had that problem with Statistics in studying mutiple regression. I had a teacher who visualized how it worked and tried to explain that way. I had to take most of that classs to my previous stat teacher who taught me Analysis of Variance and who could use words and logic to explain it to me.