I had no problem with math. English literature was my nemesis - I couldn't write a passable essay or analyze a poem if someone put a gun to my head...
It doesn't take much to 'miss the boat'. I remember the first time I tried to use paragraphs in a story. The teacher told me I couldn't do that because she hadn't taught us how to yet. Unfortunately I must have been sick the day she gave that lesson, with the result that years later I still couldn't do paragraphs because it was stuck in my mind that I didn't know how...
The subjects I did best in were those that had textbooks. I would teach myself and then reinforce that learning in class. Unfortunately some subjects didn't have textbooks...
For young students it's not an easy concept to grasp. I remember wondering why it had to be 'X'. Couldn't you use some other letter, or even a whole word? Math is so confusing...
The 'best' math teacher I had was a lazy prick. He would just tell us what lessons to do and leave the classroom. Then the good students (like me) would teach the poor students. This was great because we could get on with doing the work without distractions, the poor students actually began to understand math, the 'teacher' got to enjoy lots of free time, and we didn't have to put up with him. Everyone was happy!
In another subject I was chastised for trying to help other students. "You're spoiling your chance to be top of the class!"
But hey, that was over 40 years ago! Now we have the Internet, Google, Wikipedia. No need for textbooks and classrooms, the kids can learn it all online...
I was fortunate - I never gave a flying fece if I was top of the class!!!!!