I thought about this a lot. In college I often failed and teacher would look at me dumbfounded (followed by the traditional "it's trivial"). Very discouraging indeed. Years later I revisit some things, and without struggling too much, I get insights and ideas and overall easier time actually solving things. So in the end it does look like some "maturity" was lacking. Now it's bothersome that schools can only operate on people with magic maturity and other people are left on the side (the school as semi-comb theory) and I wish we could have some pedagogical light on what is that missing maturity and how to grow it explicitely.