I used to tutor. Quite a lot of kids missed something simple somewhere in the beginning and then were skating by having passable but not good grades. Not understanding what is taught, but getting enough points.
The tutoring consisted of going beck few years and teaching them basics again. They improved, but school never did that "maybe this person now can learn the thing he/she missed 3 years ago".
I remember “factorization” not clicking for me for the longest time in public school. My grades slowly got worse, until the right teacher came along and explained it in a way that clicked. I proceeded to have a successful math focused education in high school and university.
I wonder if part of it is that kids are at the mercy of what their teachers want to teach / the curriculum. I think kids often know which things they struggle with… they need help articulating it, and then someone to work with them through it.