cannot update payment info system is blocking it

Blocking it how, are the fields protected so that you can't update them, do you get any error messages ? Is it a debit or credit card that you are trying to use, and is the card registered to exactly the same name and address as you have on your iTunes account and you are currently in that country ?

Im having the same problem. My cvc number and expiration date fields stay blank. I fill the info in and click done. Nothing changes. When i open it back up its still blank.

When you click 'done' do you get any error messages i.e. does it accept what you've filled in ? I'm not sure whether those fields are re-displayed when you go back onto the screen e.g. whether for security they are re-displayed as blank.

No error messages. Goes back to previous screen as if it updated. Then when you go back into payment info those spots are blank again. They are usually displayed as either numbers or *.

I think that the expiry date will be shown as asterisks, and I don't think that the security code will be re-displayed either (similarly only part of the actual card number is displayed) - just in case somebody gets access to your iTunes account then they can't see your card details. If you aren't getting any error messages when you've entered them then that implies that iTunes has accepted them as being valid.