When I relax my shoulders, my hips relax and my stride opens up. Same for your horse. If you are pulling or holding the rein, their shoulders and neck tighten. You can’t get the the joints of the hind leg to articulate and come under. Relax your hand. The horse needs to relax the jaw. Lick, chew, swallow. The tongue has to be able to move. The shoulders will relax. The hind legs will be better able come under and through. You have to give to get.