One of the big differences I see between a good Tour player, some of the greatest players who have played the game, and the average amateur player is the position the body is placed in its setup.
#1 Stance Mistake Amateurs Make
I see a lot of amateur players end up in positions where the spine is quite curved, the hips too much underneath the body and the chin either poking this way or being tucked in too much. From a physiological point of view, the spine is in a position where it cannot function normally. And if it cannot function normally, it is going to have some effects on the technical aspects of the swing.