The horse you train on is probably just so seasoned that your weight may be all the cue he needs. When you look toward the next jump he feels the weight shift and adjusts.
When I am trying to teach a horse a flying change, I use a circle to encourage the switch over. I also lift on the inside rein to elevate that shoulder and push with my outside leg to shift the hips. It takes lots of practice and balance on the horses part. It also takes precision timing of the cue. You might want to start with breaking gait cueing for the correct lead each time until you get the hang of cueing leads.