I agree you should be on a 3 loop (in a standard arena).
Something to try (to get the horse prepared for something new to happen when you cross the centerline (not at A & C, just when you change bend)) - exercises that do something extra at that poin for 3-5 strides. All of these will have him listening to you at the point of crossing the centerline:
- Trot the serpentine, but ask for 3-5 strides of walk as you change bend
- Occassionally, stay on the circle instead of changing bend (on all 3 cirlcles and in both directions)
- Walk or trot (maybe walk in warmup and later do the same in trot), asking for 3-5 strides of leg yield (toward the rail) after you change bend
- Canter, but ask for 3-5 stridews of trot or walk as you change bend
Believe it or not, the "simple change" in dressage is canter-walk-canter. If you haven't done it, or aren't good at flying changes, starting them on the diagonal is better than on a serpentine. Canter around the end of the arena, and start across the long diagonal. At (or just after) X ask for a trot (or walk), then pick up the other lead as you get to the next corner letter. This makes it obvious to the horse which lead you should be on - as opposed to the serpentine where, if you are doing it correctly, you ask for a chage while facing directly at the rail.