Hi gp193,
There’s no way for me to really know this based on “theory at a distance” because I can’t see you on the mattress or feel what you feel but based on speculation it would make sense to me if you are feeling the “symptoms” on the softer side but not on the firmer side. If you were feeling the symptoms on the firmer side then it would be more likely to be pressure related because firmer layers are more likely to cause pressure issues while softer layers are more likely to cause alignment issues.
If the firmer side works well for your hips but isn’t thick/soft enough for your shoulders then it may be worth trying an extra inch or two of latex as a topper which would allow your shoulders to sink in a little more while still being able to have a firmer support layer.
The deeper layers of a mattress are for primary support which “stops” the heavier pelvis from sinking down too far while the upper layers are for pressure relief and are meant to “allow” the lighter shoulders to sink in far enough to relieve pressure and also to fill in the gaps in the sleeping profile (generally the waist on the side or lower back for back sleepers) which is more “secondary” support.
Some of the other factors that would be worth considering so you can rule them out as a possibility are in post #2 here.
The first step though is always to talk with the retailer or manufacturer where you purchased your mattress because they will generally know more about the mattresses they sell and the options they have available and would also have more experience in making suggestions for their customers that had purchased the same mattress and were in similar circumstances as you.
Phoenix