It could be an ineffective teacher. My first year of algebra in 8th grade was very difficult, and I barely passed the class. I took algebra 2 the next year when I started high school, and everything clicked. I totally aced the class, and then aced geometry the next year and trigonometry the year after. Calculus in senior year was more challenging but I think I had an A- in that. The math teacher I had in high school was apparently really good at teaching in a way that made everything make sense. When I had calc 2, 3, and 4 in college, those were also taught by very good teachers, and while it did get progressively harder, it still made sense.