Do it harder... lol. If you bop him and he "mouths" afterward, by licking and chomping his lips, then that's the signal that he's learned something. You just have to learn what the right amount of strength is. Some horses are super sensitive and will stop with a little amount, and some horses (like my dead-head), I'll have to bop him hard enough so that he'll physically move his front end away. The trick is if you do it real hard once, they won't do it again. But if you do it too soft several times, they'll think it's a game. Silly yearlings.