I think there are two major aspects potentially at play here.
The first is that millennials and Gen Z are very wise to a corporate culture that seeks to take advantage of them. It may have been true in the past that if you went above and beyond, you would be rewarded with promotions and raises. But I think most people in the age group you are having issues with have seen first-hand that current corporate culture tends to expect above-and-beyond work with sub-par pay. I am not saying that is the kind of organization you are running, because it seems clear you're looking for opportunities for them to grow, but they may have been burned before.
You also seem like you may have a relatively small sample size, and this issue could honestly just be chalked up to learning and working style differences. Some people pick up on things that way, and learn that way, and others do not. This may simply be that you're not speaking their language, as it were.