i mean if your shoulder hangs low your need to str your upward rotators in the shoulder girder I would work on lower trap upper trap and serratus anterior work. you need to str the whole back so start off by doing basic rows. you can also research downward depressed shoulder blades. also stretch out the pecs minor and your rhomboids also contribute to the shoulder to be low. this does should like a muscular problem that can be easily addressed by str training.