How can I breathe more comfortably when sitting?

When I am lying down, my abdomin fills easily, but when sitting with what I believe is proper posture, it feels like my diaphragm is stuck and my breath cannot expand into my abdomen. Only my ribs and upper chest expand. It is not caused by anxiety, nor does it cause a lot of anxiety, but is very distracting and uncomfortable. It this normal or is it something I can improve with exercises?