>>678239A) straps not fitted to you
B) to much weight for the frame
Normally the frame transfers all the weight to the frame which transfers it to your hips via hipbelt and in the end there should be only little to no weight on your shoulders, but all on your hips.
Now this is the theory. If you have broad shoulders/chest (doyouevenlift), you might need at pack that fits you better (broader back, S type straps). And if you overweight the pack in relation to its frame, you gonna get the same problem.
Check what weightclass you 'll need, go to store, ask about packs in that class, try different loaded packs, check which one fits you best. You can get pretty reasonable prices if you buy last seasons gear (which is mostly the same as this seasons gear, just a different color).
If you need milspec and also need to haul heavy, there are several European makers that have internal/external frame packs for military use (Savotta, Berghaus, Bergans). In most situations you are better off with just a regular hiking pack.
