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.
I know a strong grip is needed in all pulling exercises, so when is it a good idea to use straps?
This comes down to preference. Straps are not great if strength is your main goal because it is just extra help you are giving to your lifts so you do not get a true indicator of total strength. Straps will take away from your forearm and biceps strength, which is crucial on so many lifts. If you ever want to enter a raw lifting competition and you have done all your training with straps, you will be in for a surprise. You should train to mimic how you might compete, which is strapless. On the other hand, straps are fine for strict bodybuilding purposes, in that getting a few more reps on certain lifts can help you target muscles beyond failure, which is want you want. So again, it depends on your goals. If pure strength is your goal, then it is advisable to stay away from them. If you are looking to bring up bodyparts like your back, then occasional use is fine. Just do not become too reliant on straps.
Here is a workout which focuses on the arms. This workout will leave you with a pump.
Concentration Curls – 5-6 sets, 12 reps
Wide-Grip Barbell Curls – 5 sets, 12 reps
Hammer Curls – 5 sets, 12 reps
Standing Bent-Over Concentration Curls – 5 sets, 12 reps.
One-Arm Overhead Extension – 5-6 sets, 12 reps
Pressdown – 5 sets, 12 reps
One-Arm Pressdown – 5 sets, 12 reps
Rope Pressdown – 5 sets, 12-15 reps
1) Stand erect holding a dumbbell in each hand, to your side. Ensure palms are facing inwards and arms are straight.
2) With elbows to your sides, raise one dumbbell until the forearm is vertical and the thumb is facing your shoulder.
3) Lower to the original position and repeat with alternative arm.
The biceps may be exercised alternating (as described), simultaneous, or in simultaneous-alternating fashion. When elbows are fully flexed, they can travel forward slightly allowing forearms to be no more than vertical. This additional movement allows for relative release of tension in muscles between repetitions.