Original Detour Day Pack's Back Panel

I've just received Original Detour Day Pack (waxed canvas one) but I am not happy with how the back panel is sewed. It keeps "sinking" inside about few inches. I was expecting back panel feel to be like Commuter Laptop Messenger... I've used it for a few days but could I possibly get an exchange? Or is this something that goes away if I keep using it?
 
sad I’m confused
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