I think your not pushing it in all the way, you have to really push it in, your probably hitting the clamps/claws and thinking it is stopping but you need to push in more past the clamps inside the nozzle.
I disagree with chin or other hand motion, I had a very aggressive male panther and to teach him like you do dogs i rub under his mouth to show im not scared of his mouth being open. He shuts it and stops aggression when done, now he gets it as a good job type rub and people can say otherwise...