There are many questions about humans.

I apologize for the silly question! I just started playing. For some reason, retraining doesn't work for my heroes (or maybe I'm just doing something wrong). I have the skill that allows it, but the retraining window doesn't appear. How is that possible? For example, I have Eliza in the castle, she's a fully trained trainer, and I've built training halls. When I hover over the units, it says she can't train them. Could you please explain to me, step by step, what I need to do? Thank you!