I would also suggest doing more work at the trot. Do lots of long trotting and steering. Don't just do the same thing over and over again. Make it unpredictable. I had the same problem with my last horse, he wasn't responding well in the canter. When I started focusing on steering all over the place (in an arena or field) with no set pattern at the long trot he suppled up really nicely and it transitioned to the canter.