Sometimes, or maybe many times even, it boils down to lack of prerequisite knowledge. Most classes simply do not hammer down how important prerequisites are, and how easy it is for students to get stuck simply because they haven't read enough of the required topics.

Of course - if you are a very high-IQ individual, you sheer intellect can get you by. I know such people, that have been able to solve problems without previous knowledge, simply by going at it with logic and pattern recognition.

But if you're average, that's not going to be the case.

I've been a TA, and seen students jump from severely underperforming, to becoming A students - because they finally understood something / had the "Aha!" moment in something which kept them back.