Un-healthily skinny teen

Patient: Please help me ! I am a fifteen year old boy. I am 5′ 11″. I weigh 130 lbs. I have had the SAME size arms since i was 5 years old!! Nothing i do seems to help me gain bulk! I even tried 8 months of intense physical training to no avail. I always get picked on for how skinny I am…even though i am pretty tall for my age. My arms have not grown in width for as long as i can remember. I feel so weak and small! Why can’t I build bulk?