Intermediate casters often have developed bad habits which have become ingrained. Call it muscle memory. They aren't thinking about it....they're just doing it. That explains why it's so hard to teach them something. It's why it's harder to teach a man who's been spin or bait casting all his life (and why it's easy to teach a woman, because she probably hasn't had much experience spin casting or throwing a ball)

They may hear what you're saying but their brain / muscles have already imprinted the bad habit. The best way to break it...take the rod and stick it in their OFF hand... ie, Left for a right hander. It throws them right off and they have to learn the basics all over again. After they get what you're trying to get across in the off hand...move them back to the good hand and go from there.

And like stated before...fishing isn't fly casting practice. If that's the only time they cast a rod...they won't get better. When you're fishing, you don't think about the cast. Casting practice is essential if you want to build good habits. It's what teaches the brain and muscles so when you do go out to fish, you don't have to think about the cast. You just do it.