Because the ball is lying on the ground to the side of you there must be an upward and downward swinging of the arms combined with a rotational motion of the body. The shoulders rotate on a shallower or flatter plane than the arm swing. The tilting or rocking rather than rotating of the shoulders forces the arms to swing predominantly upward rather than upward and around. This bad shoulder movement on the backswing prevents the body from unwinding freely on the downswing. So remember to rotate the shoulders on the backswing.