The shoulder blade can be out of place which causes the shoulder joint to not work properly. Which can lead to shoulder pain.

Pain in the right ... feel this stretch in the back of your right shoulder. To do it: Gently pull your right arm across your chest as far as you can. Use your left hand to hold the upper part ...

Although there can be several origins of pain that presents itself in the front, back and ... within the shoulder, connecting the upper arm (humerus) to the shoulder blade (scapula).