And more time for it to be mulled over. It was frustrating, but in college I found that the 16 week semester was rarely enough to actually learn something, only enough time to commit it to memory. It was often a year or more later that I'd have a sudden eureka moment while studying a separate topic. Perhaps something that was applying the unlearned material (like Physics I and Calculus, which were a semester apart for me since I took Chem I my first semester in college) or something related but not exactly the same (like Concrete Mathematics by Knuth et al. leading me to a realization about some aspects of calculus).