I don't understand why my grades are so bad?

I study all the time, literally more than ANYONE ELSE I KNOW, I mean I read the chapter, rewrite the entire chapter, do all the review problems, check my answers, make flashcards, use review materials, highlight and underline stuff, give myself mini-quizzes, everything, seriously. But I can NEVER get higher than a C on a test, and that's if I'm lucky--usually I get around a D. I thought I was just stupid but then I got my IQ professionally tested and apparently it's141. I don't understand. If my IQ is 141, and I'm studying my *** off, how can I POSSIBLY be getting such bad grades? I just don't get it. What else can I try?