I was grappling last week with a guy and I had his back for about 4 minutes of the 5 minute round. I was going for the rear naked choke, but he tucked his chin and shrugged his shoulders making his neck disappear. As a newbie, where do I go from here? Is there a maneuver to expose the neck if they pull off this defense? Is there another sub I should transition to? I have tried looking up vids on youtube but all the subs are done on a compliant opponent so they don't explain how to work the choke if guys do stuff like this.