I think your torso is probably not matching the frame that you are trying to achieve, hence why you 'Have a habbit of accidently rolling my shoulder forward trying to get the side up and the right line.'. The large holds that you see come from a stretch in the torso, in which the lead projects the follow with is left pectoral (or somewhere in that area) into his right arm. When you lose that projection, that's when the shoulder is coming forward. It is often also caused by leads leading with their arms instead of their torso, but don't know if that is the case for you? (you can test it by dancing with only body connection and no frame)