I am not a trainer by any means, but did have this problem. My horse used to pull his face to his cehst, the ground, and completely ignored me in his snaffle. In harsher bits he would do it less, but not stop. I tried lessons, a trainer got on him, had the vet out,etc nothign stopped it. ONe day I was sick of my hands hurting and yanking back on his mouth, I looped the reins in my hand and smacked him on his neck/wither area. He popped his head up. Everytime he tought about it, I popped him, and eventually he stopped. It probably took 2 days.