No backpack fits me

Hi,

I would like to buy a backpack. I have tried a few of them including  Granite Gear (Crown, Aji and Blaze) but no luck. I have a strange problem. For some reason the shoulder strap does not contour to my shoulder. There is space towards the back under the shoulder strap. I have this problem with other brands as well. It is very frustrating. Would you know what is happening? At first it would seem that the   pack is too long but I am already trying the short torso packs. The waist belt is resting on the hips. I had people help me at the stores. I am 5"6' so I am not petite and my torso looks proportionate to my body. I am really puzzled. Has anyone come across something like that before? 
I am looking forward to hearing from you.
Thank you in advance.
Alina