Buttstock sliding when shooting (Fixed)

[lazyengineer]

Something looks stuck in your picture above. The little oval with the cross-pin visible on the left. Every adjustable stock I have with a similar arrangement has that cross pin at the TOP of the oval when the locking pin is in a hole on the buffer tube.

Try pulling it off the tube, does it go to the top? If so there is likely a size difference between the locking pin and the position holes on the tube. If it still doesn't go up then maybe an assembly or binding issue with that locking mechanism.

Edit:
One other thought, does that model stock have some sort of locking mechanism intended to hold the adjusting lever open or closed? When all else fails read the instructions.

This. Something is blocking your pin from going all the way up, as it's not supposed to be sticking out like that; unless there's something unique to how that particular brand does their stock. Either replace the stock and try a different one, or see if you can pull it apart and adjust with a file whatever you need to, for it to more freely allow the pin to go all the way up and into the stock. Maybe the pin diameter is too large.