I can't gain any weight?

Everybody in my family has the same problem. I'm an athlete so I know how it feels. I'm constantly in the power lifting weight room. Are you doing lighter weight with more repetitions or maxing out. To gain weight while exercising you need to max out. This builds bulk. Lighter weight/more reps just tones the muscles. Do heavier weights and keep with a protein and calcium rich diet and you should put some weight on.