I've never had Hilton gift card but are they the kind that you can go on the site and register? My company runs hundreds of websites and people trying to buy things with gift cards all the time and that's one thing that we suggest. Since the sales are online, there has to be a match of some sort of information and if they're not registered there's nothing to match. Not sure if that would help with yours but if you can register, I would do so

I don't know if I'd be surprised if they didn't work internationally though. A pretty good percentage of gift cards purchased in the US do not work outside the US.