Problem getting shoulders back far enough

I think I got the "hang" of stretchsitting as I can sit and get the stretch in my lower back. My problem is that in every chair I've tried, I'm not able to get my shoulders positioned back far enough. I understand how to do the shoulder roll and get them back, which I can do standing or sitting on the edge of the chair. But when sitting on a chair with back support, my shoulders are obstructed by the chair back. Any suggestions?