Did you allocate all your favoured enemies? There might be a fun bug where it doesn't work if they aren't allocated (even though it changes automatically). I say this because when I click on your hero profile in combat I can't see the favoured enemy line.
And I purposely played more than 15 turns in this battle without any magic.
And tried using rain of arrows talent of my hero but still it just misses it's chance and does nothing.
