Why cant I use my apple ID balance that I loaded onto my device with a giftcard

I redeemed an apple digital gift card onto my sons Ipad as I have many times, however for some reason he is not able to make any app store purchases with this one even though he has funds available. It simply tells him he has no payment options. I have tried adding my debit card which only bills my debit card and not his apple cash. Please advise or tell me how to get my money back from the giftcard.