I guess it has to do with the affinity, but I don't see how this is possible. I have no affinity with anyone other than Prince Serin, with whom the affinity is quite low. I don't believe I've made so many wrong choices as for this to happen. It's definitely a problem with the demo. There is no reason for it to have worked in the demo and not now.
Thanks anyways! Guess I'll be playing again.
One question though: is there any practical purpose in teaching the princes elements outside of their affinity?