In college I struggled with Calculus. I'm no dummy, I score as a genius IQ on those kind of tests. Yet Calculus and I didn't get along.
As a computer science major I was required to take Calc I, II and III though for some reasons the fourth semester (Differential Equations) were not required, for me. I ended up taking each of the I, II and III semesters three or four times each. Yup, a genius IQ and I have to retake core cirricula like Calculus several times.