I have started riding this really nicely trained dressage horse that a friend owns. The last couple of times that I rode him I was just doing the intro level dressage tests to keep us both busy. However the problem is that when I go down center line I am suppose to halt through walk, but Magic won't go down to the walk and then halt, he just goes from the trot to the halt. He will go from the trot to a walk then a halt anywhere else in the arena, except for when I go down center line. Any suggestions?