This medium trot needs to come up off the forehand and have more engagement from behind.
To get your medium trot off the forehand and coming through more from behind try the following exercise:
On the long side of the arena start in shoulder-in. About a third of the way along straighten up and ride 4-5 strides of medium trot then return to shoulder in. The shoulder-in provide collection, lightens the forehand and engages the hindquarters and makes sure that the inside hind leg is stepping right under. Make sure you are back in shoulder-in again before the medium trot loses its quality. 4-5 strides is plenty to start off with, you can gradually increase this as the medium trot improves.
