I have found that clothing makes my packs slip, especially polyester. I lift up my shirt before tightening the belt so that the belt and lumbar support is directly on my skin. Pack slip happens more for me during the summer training walks when it's hot. The fabrics seem to heat up, absorb more moisture, and stretch more. When I get in cooler weather I don't notice it. I don't like to have the shoulder straps touching my shoulders and so this causes me to want to ride the frame higher than it is probably designed for or needs to be.