You can get more players to get the right shoulder on plane by telling them to move their left shoulder up/back then telling them to get the right shoulder down plane.
James,
That is a sure fire recipe for round housing.
The left shoulder should not take any active part in the downswing and must only be left to respond to the dictates of its right sided cousin. If "righty" goes down and out then "lefty" has no where else to go other than up and in. Not so the other way round though!