Amazon won't let me buy items?

I have a gift card loaded onto there and it says I have more than enough, but it keeps telling me to put in my payment information? I'm trying to ship the items to Australia if that has anything to do with it, but it does this when I'm trying to send it to the us (my current location) too. Sometimes it lets me pay and sometimes it doesn't. Why? And how can I fix this?

2017-04-11T00:37:33.809Z