What is your diet like? Sodas, sugar, yeast, gluten, etc could be the cause. Kefir is really good for acne, lots of water, fruits, vegetables and perhaps some kind of exercise.
I used to have acne as a teen and it's a pain in the neck, not to mention my diet was horrible. Do you eat lots of breads and sugary stuff?
Also, I've heard of doctors prescribing birth control pills for patients who have acne with remarkable success. Sometimes it's hormonal. I would (this is just my opinion) evaluate my diet, then if that isn't the problem perhaps see an endocrinologist if the dermatologist doesn't resolve this.