Sorry I haven't been able to replicate this since I'm playing on Ironman, but here's what happened:

I finished training my second leader to her first rank, but decided not to continue training her for the time being. When I did go back to train her for rank 2 she was unselectable. She wasn't wounded or anything. I was still able to train my first leader, or to select new leaders, but leader #2 has remained untrainable.

anyone else experience something like this?