I had this problem when I was trying to purchase directly from PA website for a trip from PVG to MNL.
My Capital One Visa wouldn't get processed. And calling PA didn't help. PA just said my credit card was declined. Capital One said that it was not a problem on their end. Everything looks fine.
It turns out that the credit card processing from PA requires the "Verified by Visa". This is an extra security step from Visa that requires you to put in a password that you set up.
My Capital One card doesn't participate in this program. So if I shop from any site that requires this extra verification, I can't use the CapOne card.
I ended up using my BankofAmerica card, which does have this feature. Everything went through fine. Although I had to pay the 3% international usage charge.
The same thing appllies when shopping on Singapore Air website. Their CC processing requires Verified by Visa.