The reason some kids are not motivated to do math is because they believe they are bad at it, and nobody likes to do something that makes them feel dumb or not good. The kids that love math (like myself - I remember I was SO excited about math as a kid) are good at it, and teachers are constantly complimenting them.
You can hack the brain to feeling good while learning. I just need like 200M but I might be able to do it without.
I don't mean to be negative, but can you name a research paper where a normal students 7 hour school day was changed to formative assessment all week long for a semester or more, and the grades improved? Or identify why students on Duolingo don't tend to learn as well as those taking in person classes? Why does the effect of the Testing Effect/formative assessment tail off when it is used more and over greater time periods?
The point is, the research on learning methods usually researches them in isolation and in 5-15-60 minute study sessions. Maybe doing 15 minutes of formative assessment improves your grades, but puts you off doing other work or makes you less effective in later lessons, when done regularly. There is a lack of useful longitudinal research in Education for the depressing reason that the two main effects are your genetic background, and private teacher attention.
I don't think kids are not motivated about math because they are bad at it. It doesn't seem like a subject that most kids see themselves using every day, because the adults they see around them don't seem to (and in fact largely don't) use Maths. While I value and enjoy Maths, and know how knowing maths makes a massive difference to your experience or life, kids can't, and really won't have good reasons to understand or be motivated through that. e.g. it's not a fear of negativity (though in class under pressure it is). It's an absence of motivation in the first place.