Duels always hang at "building the match..."
I'm trying to start a duel with one of my friends. Even if I'm quick enough to tap the game center notification that flashes at the top of my screen when they invite me (and how do I accept the duel if I miss that 2-second window?), it just takes me to a screen that says "building the match". We've tried several times, but it never gets any further. Any ideas?