A common cause of that is not getting a good cheek weld. The cheek should press firmly on the stock ( but not excessively so) or you get a hammer hitting a nail effect. It also helps in keeping the shooter from raising his head as he pulls the trigger and promotes a consistent mount. It's hard to tell by observation.
Could be a stock fit problem but a form imperfection is more likely.
Yeah, they can be stubborn. I've spotted form problems where a kid just wouldn't change. After sufficient nagging they'll give in to get that jerk instructor off their back. Then you get a wide eyed look like maybe the old fart knows something when they begin to hit better.
Hint: Some kids get a streak of defiance about that age. Had a kid who was coached heavily by his dad and the kid would do pretty much the opposite. Finally his dad backed off and I told the kid the same thing in a different way and progress started to be made.
