The mouth is never the real problem. What you feel in your hand is a reflection of what is happening behind the saddle. Get the back relaxed and swinging, and the hind legs active, and the horse will feel like putty in your hands.
The key to success with any movement in the Dressage ring lies in how you ride the corner preceding the movement.
In a good, productive stretch - active energy travels over the horse's topline and into the rider's hands. If you don't feel at least a little power coming from behind up into your connection, your stretch is not correct!
“You need to produce a walk. So work at the walk as well as the trot and canter. Get a good feeling of the body working in the walk. The walk is a mirror of the training of the horse.” ~ Christoph Hess
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.