I used to tutor folks in basic math classes at a small community college, mostly classes leading up to college algebra (Math 097, 098). Varied backgrounds, ages 18-50s.

There was a common theme to some of the problems these students had: if you didn't quickly grasp a concept in the specific way that particular teacher taught it, the class would soon move on to the next topic anyways. And sometimes teachers would not teach one particular thing very well, but it was something the rest of the class's instruction depended on. This left people adrift for the rest of the curriculum. If they took the class over again, they could get stuck at the same point because the class moved at the same pace, often with the same instructor. That's both incredibly frustrating and a huge waste of time and money!