What Causes Persistent Stomach Ache?

Hello doctor. My daughter is 17 yrs. old. She has been getting severe stomach pains from 1yr. Done with all kinds of tests. Everything is normal as results . Went thru Endoscopy too. Even that result was normal. Then why still this pain. At first it started as monthly periods pain that would last for first and second days but now it s almost every day whether it s her montly periods or not. There is no single day gone without her feeling the pain. No doctors are able to help her. What can this be. With so many good doctors around and so much advancement in science and technology no one is able to help us. What can be the problem. Can u help us here. Please.