During the bench press I do my best to pack my shoulders down and in to prevent pain/injury (I didn't know about this when I first started working out and experienced some pain during this exercise). However, sometimes in the middle of a set my right shoulder "unpacks" and I start feeling pain. My two choices at this point are to either finish the set through the pain, or to stop and readjust my shoulder and then finish the set. Obiously, neither of these are ideal. What could I be doing wrong? I've tried to squeeze them down and in as tight as possible to prevent this but it still happens sometimes. For some reason this seems to only happen to my right shoulder. Any advice?