Cannot swap between characters in graden/graveyard ops. Pvz gw2 xbox1

Cannot swap between characters in graden/graveyard ops. Part way through playing solo ops I'll try hitting up or down and it won't work I'll be stuck on whatever character I'm on even if it's not the main/top one I've chosen. I've tried shutting down the game and it works at the start of the op then stops again.