One of my great fears is putting in the study/time and getting no results.
Some people are good at some things. They pick them up quickly, flesh out their skills quickly, and apply those skills well in an almost intuitive way. You can call this "intelligence," but some people will be very intelligent in other ways, and have no aptitude for a particular skill.
Maybe the aptitude is innate in your biology, maybe it's the result of nurture in your childhood, maybe it's even somewhat learnable as an adult, I don't know, but when you teach people things, some of them pick it up and apply it well, and others are much slower.