I'd recommend starting your shoulder workout with a couple of power lifts, such as any combination of military press, push press, clean and press, upright rows, ect.. Picking a heavy weight, trying to hit around 6 reps, give or take. Then follow up with some isolation work such as side DB raises, rear DB raises, ect.. You won't' be able to use a lot of weight if you're really trying to isolate the deltoid heads (any straight arm raise), but you're making the deltoids do most all of the work, which is what you want anyway. Pick a weight that allows you to do around 8-10 reps or so, really concentrating on contracting the muscles and having to use every muscle fiber you have!
That's what works for me anyways. If your shoulders aren't responding, you'll have to increase the intensity somehow; increasing weight or maybe having to change the way you train them completely. If the rest of you is growing, and your shoulders aren't, then it shouldn't be a matter of diet, but rather how you're training them and what works best for you. Trail and error my friend.
Good luck bro