What causes payment decline and how can we solve the problem

There are several possibilities...of course, it could simply be that your credit card is out of date or out of credit.

It can also be that it's in the wrong currency - iTunes and App Stores will only accept credit cards in the local currency. If you need to purchase apps etc from another App Store or iTunes Store, then you can do this with an Apple Gift card in the local currency - say, US dollars, for example. Even if you're not in the US you can purchase these on-line. You only need the code from the rear of the card, so the retailer can email you this immediately and you're good to go.

Tim

I think the most obvious is that you maxed out your credit card... Or that your available balance is not enough for the purchase price... It could also be that the card was cancelled (by the credit card company) or the credit card being used is not a recognized brand by the vendor.

This is an old thread which was resolved by PM so It is not much sense responding to the OP at the same time we appreciate your input.