When I input code it just keeps asking for iTunes account password over and over again not loading card

I got a £25 iTunes gift card for my birthday and I have tried to redeem it but it is not working. I don't know if it's not activated but there is no message saying that the card is not inactive and I have checked all the letters and numbers and I have entered them correctly. When I enter the code it asks me to enter my iTunes account password I do that and then it just brings up the box to enter the password again over and over again... Hope someone can help! Thanks!