Your shoulders have a lot of little muscles that do a lot of work, but your back has larger muscles that help move the shoulder blades and the arms. When you move your arms think about the motion coming from the back muscles instead of putting all the pressure on the shoulders.
Lie down on your stomach with your arms out to the side like you are making a “T.” Exhale and lift your arms slightly off the ground. Feel the motion come from the upper back between the shoulder blades. Keep the motion controlled. Don’t just flop your arms up and down; glide them up and down.