I too have had a similar issue. It started when I was in High school, playing football. I tore off some cartilage in my shoulder. I started training again, heavy, to early in recovery and paralyzed my left side thoracic nerve. I never followed the physical therapists orders, and the small muscles around the area began to become very weak. I don't have much control over my wing bone. Its really thrown off my left side.
I recently started back to training about a year ago. Alot of time spent doing benches and other compound movements, but neglected my back, and especially my rear delts and surrounding muscles. Which has made that area very week. To the point where it hurt to go heavy on presses, and pretty much killed military presses for me. I started doing them again with dumbbells, going very light and just focusing on form and posture.
I've also been hitting my rear delts a lot more, as well.
You might think about going to see a doc. Or even talk with a physical therapist.
I've learned you can't neglect the little guys. They'll make you pay.