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.
Heavy incline barbell presses are a well-known chest-builder, but they can be dangerous if you are training alone. Done in the Smith machine, they will hit your upper pecs through a full range of motion with less risk of injury. You can work in some partials to train weaker parts of the movement. Finish off with a set of incline presses with a huge drop set, removing plates and getting right back under the bar each time you hit failure. This technique flushes the muscle with blood and takes you well past failure. Partials and drops on the incline Smith should help your chest a great deal.
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
– Taking it out of a power rack or picking it off the floor and carefully cleaning it to shoulder level, stand erect holding a loaded barbell at shoulder level with a grip well outside your shoulders.
– In a smooth, strong motion, press the bar straight up to just short of elbow lockout.
– Squeeze, then lower the bar under control to the start and repeat for reps.