We once had a person in our group. He never attacked me (nor anyone else) with any effort. Not even my Gi would move if he'd hit it. It turned out that he wasn't all that keen on practising, but it was his father that wanted him to practise. We don't take studens younger than 15, and this guy was probably not a day over 16. He never said much and was very timid. My guess is that he was very "hard kept" (can you say it like that in English, or does it make no sence?) by his father... Eitherway he quit after a year.
I have no deep thoughts about this subject, but perhaps there are sometimes more reasons for a persons behaviour, than meets the eye.