Any nutrition experts out there? Or personal trainers?

Hi, I'm 23 about 5'9 and here's the weird part I only weight 8 stone!!! I need to put on weight, I'm not exactly skin and bones kinda lean but I would feel much better if I could put on some weight not even for physical looks. My main thing is I eat and eat and eat and eat but I just don't put on weight... am. i eating the wrong foods I'm just eating junk food my thoughts being ok people's main overweight issues are usually junk food so if I eat lots of that I'll put on weight but nope
... thanks
Any nutrition experts out there? Or personal trainers?
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