I used to be a good squatter but for the past few yrs I've been getting knee pain doing them and cant really seem to feel quads. I'm used to low bar placement(between trap and rear delts) but lately cant seem to get in that position and the bar moves up and to the side during the rep. my rear delt where the bar normally seats on doesn't "pop" out and feels like the bar will slide down. I also notice severe wrist pain on my left side and excessive rounding in thoracic spine. I feel many of these problems will resolve itself if I can get the correct placement of bar on my back. I also seem to have trouble keeping lumbar neutral or arched on the bottom of sqaut and deads. any suggestions?