You have taken candidate B and idealized him much more than I think is fair. You have made him into a fledging version of A, by assuming he can be taught to the roughly same level eventually, but without the attitude. You then offer a backhanded jab by saying if B is unteachable to nearly the level of A, it is the teacher's fault. This is an unfair conclusion to jump to.
A positive attitude and a brilliant teacher aren't all that is needed for training to be successful; you also need a capable and curious student. They have to be able to learn this stuff, and they want learn it for reasons other than pleasing the teacher. Where these qualities aligns, the student is bound to do some learning on their own; unless they elect to learn the exact same things as everyone else, they are bound to have something to teach others. That the OP mentioned no one really learns anything from candidate B suggests he doesn't have the drive and curiosity; these are warning signs.
Furthermore, you can't teach creativity. You can't provide someone with a list of steps to follow every time they want to create something new. The list represents the creative product, and the list is just someone following by rote.
What you can do is sow the seeds of creativity; you can inspire. You can ask questions to help guide someone's thinking, and get them past mental roadblocks. You can throw in ideas and expose flaws. But you can't teach someone to be inspired and come up with new ideas. You can't draw them a map, and then expect that with that training alone they'll be able to draw the next one. You have to rely on their brain's capability to form new connections on its own.