The OP stated the girls brother has shot the gun with no issues. I'll stick by my assertion the girls form isn't providing enough resistance for the gun to function. Again, that can be grip, wrist, elbows or shoulders. She needs to grip the piss out of the gun and push out HARD.
Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats.
| 
