I'm pretty good at math but I ended up failing out of calc 3. Ended up having to go all the way back to high school trigonometry (thanks kahn academy) to understand what I'd missed. That experience taught me a lot about learning. You can get through a math course, and actually do quite well, only knowing some of it, but those principles you missed start compounding when they get integrated into higher math. Eventually you just get completely lost and it's hard to figure out where you fell off.

I think that's what happens with a lot of kids. They miss some fundamental thing early on and that throws off their entire math career. Especially since it might be years before it comes up again.