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Question: Ipod 6th generation 120gb classic

I just bought a refurbished Ipod from a reputable dealer for my sons' birthday. He has downloaded some songs and maybe 10 TV (1/2 hour) episodes. When he tries to download more (TV) the Ipod erases some of the older ones. He has , I would assume he has plenty of memory left (120GB seems like a lot). I had him when I was 50 and he is 13. I downloaded the manual but of course it makes no sense to me ---- computer illiterate boomer. Is he doing something wrong or do we need to return it ? The seller said he would take it back but I don't feel like rewrapping it and a trip to the post office.
Thanks for any help.