I talked to a lab supervisor (i volunteered in his lab for 8 months) and he also taught me a course, but in a class of more than 300 students. He said he will not be able to give me a strong recommendation because he doesn't know me well enough, and that i should look for someone else for the academic reference. I am really confused now because my research supervisors didn't teach me any courses, apart from supervising my thesis. Not sure if thesis supervision counts as a course.
I am really upset because I dedicated so much time towards his lab and his research.