I am fairly new to teaching and found some students simply not understand programming. Computer programming involves problem solving and some memorization and not a lot of kids are into that. Let's face it, computer programming can be a bit tedious and frustrating for a lot of people and so its no wonder why some kids don't choose a career in it. In order to be good at programming you need to have some desire and perseverance.
I find the kids that are good at math are also good at computer science. However these kids also lack a sense of creativity whereas the other kids who don't do so well in CS are more extrovert and excel in reading/writing classes.