I've had a Pioneer 36 for a few months now and think I have it dialed in pretty well except for one thing. When underwater if I turn sideways the top of the BP (near the shoulder straps) seems to slide down to that side. I don't like the feel of the BP sliding around on my back. I guess I will try shortening the shoulder straps a little but they are going to be tough to get into with a drysuit on if I do this. What am I doing wrong? When I'm u/w should the straps be tight so the bp doesn't slide around?