Doing more weight and more reps?
Okay seriously, eat more, lift more, repeat. It’s very hard to put on additional size for any one muscle, and if it weren’t - nobody would ever complain about having small arms. But if you want, this article was written literally yesterday… maybe you missed it, or maybe you don’t read the 3,000+ articles in the T-Nation archive. Either way, give this a read and you might find your shoulder cure.