Overall, I like to help you as much as possible.
If you are applying for a MS/course program, the reference letters probably are not very important, as the top US schools (to my knowledge) mainly look at your statistics, like GPA, ranking, GRE, etc.
To put it more bluntly, MS program is a source of revenue for them.
For this, all you need is to have obtained top grades in the courses I have taught.
However, I can only comment your specific course performance but not extrapolate, e.g. from basic programming (a class you took with me) to machine learning (a class you did not take with me).
If you are applying for a PhD/research program, you need to have at least some good publications. Any decent professors/researchers know that good grades do not imply good research potential. (I am not aware of any rigorous study, but I think the two are weakly positively correlated at best). Thus, I will write letters only for those who have published top research papers or built good industry products with me, as otherwise the recommendation is likely moot.
