Have u ever tried the millitary press or super sets in your shoulder workout?
You should also try to increase your weight from week to week,even if its just a little.
Your muscles will get new stimulations if you increase the weight and will grow.
Also I would recommend you to try using creatin.
It'll help you to get bigger, stronger and you'll recover faster after a hard workout.
Try using it for 8 weeks and take everyday 3-5 gram of it.
Best regards Coach Nas