Actually I'm having the same problem, kinda. The Valkyrie Vortex wasn't giving me any rage xp at all on the first island and I'd kind of forgotten that it was supposed to. Then on the second island I got Christa and the money trap ability. I used it a couple of times in a battle and I leveled up twice afterwards. So the Valkyrie Vortex wasn't giving any rage xp at all.
