I've noticed that a lot of people, when confronted with an unfamiliar process, panic and make mistakes because they are panicking. The process never becomes familiar because they're never ever able to complete it successfully because of the mistakes - and the hormones associated with stress reduce how effectively you learn - so it's a vicious circle. A lot of times they're making very tiny mistakes - pressing buttons too slowly or too quickly causes problems with a surprising number of keypads.
When I need to teach someone something and they're messing up a lot after a demo/verbal/written instructions, I address the fact that the person is unfamiliar with the process and they're making mistakes, and go through it step by step. The hand-holding seems to help a lot.
There are certainly people who are so high-strung they can't calm down enough to learn things, and some people who simply don't care to learn them.