Originally Posted by Kayty
How were you asking for the rein back though? Every rider I've seen try to rein back by moving the reins around has had a resistant horse. If the OP's horse travels fine and comfortably with a contact at other times and will pull up with the rein, I'd be dubious in saying it's the bit.
I would pull back with both then whichever back foot went back first I would release the pressure from the rein, and so on. I changed his bit and he backed wonderfully after that. I actually haven't seen him throw his head once since changed.