Suggest Remedy For Flat Pectoral Muscle On Chest

hi im a healthy 30 year old male. my problem is that only one of my pectoral muscles seems to work. my left pec is fine and looks normal but there is very little muscle around the right pec infact it just flat. ive been using weights for years but it never seems to get and better! now my chest looks so strange with one pec big and the other flat! please help me is there anything i can do?