I am wondering if you are trying to hold the horse's head down in a "frame"? If the horse's head is held down and behind the vertical , then it becomes hard for it to step under. Even "Long and Low" riding, used in dressage training, has the horse stretching its' head forward and outward, not coming behind the veritcal/bit.
So, I would open the horse out forward, encourage it to let its' head come forward, and put some energy into his hind end. don't lope, gallop. free him up and move him out, and don't worry abou the head. Really go for a bit, and when you come back to a lope, if the horse starts to drop behind the bit , push him up faster again.
I am NOT a fan of Mark Shaffer and to be honest, I watched several videos of his training and found that the beautiful sorrel mare he was working on moved better in the early part of the training than after him working on her for months. Think that will start a firestorm?
It's always a huge debate, the whole Western Pleasure headset and funky moving stuff.