itunes purchases are not using my redeemed gift card...how do I get this to happen?

I redeemed a gift card several months ago (perhaps years ago) and I always assumed that purchases were coming off of that balance until recently when I noted on a receipt that they were being charged to my credit card that is on file in my account. I have read through other recommendations and I cannot delete my credit card to force the purchases to come off of my gift card. I get a message that says a credit card is required because of Family Sharing. How can I get purchases to use the gift card that is in my account before charging my credit card?