For any muscle to grow, you have to work it hard, and let it have enough rest. That's sometimes hard with the shoulders, because you also work them when you do chest and back work. So if you do a rotation where you do back day 1/ chest day 2/ shoulders day 3, you are actually working the shoulder 3 days in a row.
It's also tempting to want to work lagging muscle groups more often, thinking that it will stimulate growth. Instead of working the shoulders more, go as heavy as you safely can on your shoulder exercises. And be sure to work all three heads of the shoulder: front, side and rear (though if you do a lot of chest work, additional isolated anterior shoulder work isn't really necessary, because the anterior delts are well worked during chest presses.)
I recommend doing a 2-day split (upper and lower body) like PUB. Doing each workout 2 x/week. Go as heavy as you can safely on the shoulder exercises. On the second workout, do your shoulders FIRST, after the warm-up, so you can go as heavy as you can and put more energy into the body part (this is called "priority training": you work the body part you want to improve first, when you have the most energy. Another split could be chest/back then shoulder/arms (P90X has this split, as well as a chest/shoulder/triceps and back/biceps split: P90X helped me improve my shoulders, and includse some very nice shoulder warm-ups and stretches as well).
And don't worry about not feeling sore in the muscles. DOMS is not necessary for muscle growth and strength improvement.