I saw what looked like this sort of thing with my daughter. She struggled in her high-school algebra 1 class, but whenever I would sit down with her and she would work out problems, she had no trouble (but she also had no confidence that she could do it).. She ended up having to repeat the class, which ruined her confidence even more, and she hated it even more the second time through.

At the start of middle school, her teachers were recommending accelerated math classes, but she never ended up doing that, because she stopped liking math and stopped trying. Why? Maybe because it was unpopular or she didn't like her teacher... I don't know. In any case, she went from kinda-liking math to completely hating it over the couple of years, even though when I was there, she seemed to understand it just fine.

Teachers are important. I had consistently bad grades in science until I my 8th grade science teacher that liked to blow stuff up.

I really like him and I wanted to impress him with how smart I was and I remember putting in a lot of time doing all my homework. I got my only "A" in science for the first two quarters. Then he got fired and became a harley mechanic. Back the C's and D's

Accelerated math could have been a solution -- I know that when I was bored in math I did worse, and when the problems got more interesting I did better. Now I have a math PhD, even though I got so bored in one class earlier on that I had to drop it in order to avoid failing.

There are a lot of social factors in US schools, too. I notice a lot of young women doing poorly in algebra and precalc even though they're perfectly capable if you sit down with them and ask them to simply write down the steps of solving the problem. The social benefits of being "bad at math" can be seductive in the short terms and the long-term benefits of being good at math are entirely invisible to many young women. Guys at least see some male nerds on TV making good money; girls who want to be like nerdy tv stars know that they have to go into forensics and don't know that math is very handy for figuring out the rate of cooling of bodies and decay of tissue.

Maybe she found easier math boring, therefore did low effort, therefore had bad results. It is a cycle, happen to kids that have aptitude but are in easy class. The teacher might have been right.

I used to have bad grades from classes I considered easy.

Encourage her to give it another try when she gets to late high school or college. I know a lot of math phds who fell out of love with math around middle school and then fell back in love during their first proof-based course in college.

Yeah, this.

I hated math in middle-school and early high-school. It was boring and rote.

But proofs based math, and learning how to think of math as a language, instead of a collection of overly specific "solutions" crammed into my head by teachers, was the key.

I never got proofs. I wish I did.

The thing that made it click for me was getting a "first principles" book for something relatively simple like Linear Algebra (or some basic CS-flavored discrete math book). These books prove everything, and reading the proofs the author wrote made me recognize what you needed to do to prove something.

Would you recommend one?

There are people who have dyscalculia, essentially math dyslexia, and it causes problems beyond simply being able to do basic algebra. https://en.wikipedia.org/wiki/Dyscalculia