I also at a time had this issue and a few of my peers. Thee above suggestion made by Carolyn is correct, but I do believe there is another reason that it might be failing that was not mentioned. Now, I have had a couple of peers, also including my self, going through this same problem as having to sign in and it redirectiing you to your account, making sure your billing informaiton is correct and all numbers matching. What I had figured out is, by some chance, at the point where Apple does its billing invoice for the day, when your payment method is charged, if there are insufficient funds available from your card at the time, iTunes will not finalize payment. It will then, be a pending overdue balance on your account. That is why it takes you to your billing information. If and when you put your cards security code, and still doesnt accept it, it means that iTunes is still not being able to charge for the full balance. To confirm this, going to your account in the iTunes on your PC/Mac and checking recent purchases, it will say that there is still a balance due before you are able to make ANY downloads to your devices, it being Free ,Paid App or a App already purcharsed before, iTunes will treat this all the same. Once you enter your billing information again, and iTunes then confirms payment for your pending balance, It will accept your cards security code, save it, and then let you resume to making your purchases/downloads on your account/decvice. Hopefully this helps.