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Sounds like possibly the shoulder straps are too loose.  Ideally they should flow from their connection point up your back and over you shoulders without any gaps.  the load lifters that connect the top of the pack to the front of the shoulder pads should be snug  and you should be able to tighten them and bring the pack closer to you back without causing the shoulder straps to bunch up.  Poor fit may prevent that but once you have the weight better distributed you may have better luck.