why cant i download games anymore get asked to check credit card details

If iTunes is reporting that the card is not valid, then the bank is refusing it to iTunes. You need to contact the bank to make sure that the information that you have in iTunes matches the billing information that the bank has. You can also contact iTunes support for assistance. iTunes Store - Contact Us - Apple Support

Because iTunes requires you to have a valid payment method on file, and if it needs to be updated, you need to do that. You will not be able to use the store until you update. This includes free materials.