My dear Friend, Failure to gain weight inspite of eating normal diet may be an early warning of dibaetes or thyroid disease. Please get yourself checked for it. If these test are normal, then go on high coarbohydrete and high protien diet, start working out. In stead of gaining fat aim at gaining muscles. Please do not take the weight gain supplements advertised on TV and other media, these may contain harmful chemical, that can damage your health permantly. My best wishes.