The delts are more like the endurance muscles you see with calves or forearms and they can handle more reps. Some athletes who are really gifted can do heavy barbell or dumbbell presses and just grow easily. If you have problems gaining size try giant sets (where you combine 4-5 exercises and complete them consecutively with no rest between). It is really intense and you will feel a killer pump on your shoulders. Do a giant set with dumbbell lateral raises, dumbbell shoulder presses, upright rows and bent-over lateral raises for 10 reps each exercise, or 40 reps to total. If your shoulders are not responding from heavy training, this type of advanced supersetting will really work.
Here is a workout which focuses on the shoulders:
(Standing or Seated) Overhead Dumbbell Press – 3 sets, 7-8 reps
(Standing or Seated) Dumbbell Lateral Raise – 3 sets, 10-12 reps
Alternating Front Dumbbell Raise – 3 sets, 10-12 reps
(Standing or Seated) Bent-over Lateral Raise – 3 sets, 10-12 reps
Dumbbell Shrug – 3 sets, 12-15 reps
– With a dumbbell in each hand and your chest up, back flat and knees slightly bent, bend over at the waist until your torso is just above parallel with the floor.
– Let the dumbbells hang directly beneath you with your arms locked in a slightly bent position.
– Without rising up and keeping your arms bent, powerfully raise the dumbbells up and out to your sides until your upper arms are about parallel with the torso.
– Slowly lower the dumbbells back to the start position and repeat.