You might already do this, but I'll say it for the awareness of others, too. Putting on a pack at the start of the day isn't like putting on a shirt. You can't really put on a pack and just forget about it 'til the hiking's done. (For that matter, I'm always unzipping my shirt, pushing up sleeves or pulling down...) In other words, I run across a lot of people who put on their packs and hike pretty much the whole day without making adjustments to the pack. You have to make adjustments, or your shoulders will hurt!
I probably get carried away (perhaps out of a degree of boredom) but I adjust strappage every fifteen minutes or so (whenever the urge strikes me). It's rarely very much; I might tighten my load lifters a half inch, and so loosen the bottom of the shoulder straps about that much. Maybe I'll really loosen the load lifters up and really crank down on the lower straps, getting more weight on my shoulders. Maybe I'll instead crank down the load lifters and really loosen the lower straps, getting most of the weight on my hips. But most of the adjustments are small and frequent. Making those adjustments constantly changes the pressure point on my shoulders from the harness. Instead of developing one consistent pressure point, I end up distributing shoulder harness pressure over a roughly 3" long swath that includes my collar bone.
Hope this helps.