This is something that you'd need to contact paypal or xsolla about. We don't confirm the transaction, they do. They should be able to help out if it is failing and let you know why it wasn't approved. =)
The contact info for xsolla can be found in the area where you purchase points - there is an option for live chat, and a toll-free number. If you need to contact paypal, there should be info on how to do that on their website.