Supraspinatus abducts the shoulder. Latissimus adducts the shoulder.

Perhaps your supraspinatus is all jacked up and inflamed because the functional antagonistic muscle latissimus Dorsi is inhibited. It's asleep at the wheel. Lat amnesia. 

No functional balance. Too much YIN and not enough YANG. Which one is bigger? Good hint it should do the majority of work. If you were the supraspinatus you would be really pissed off too. How does it tell ya that it's pissed? PAIN! Don't blame the supra. It's punking you out. Go after the lat. It's probably offline.