Unable to purchase 'Whatsapp' on iPhone 4

Hi there. Sorry if this question has been asked before, but is anyone having trouble purchasing 'Whatsapp' on their iPhone's? It keeps telling me my card has been declined, which is rubbish as I am able to purchase other apps, no problem. It's really frustrating! Any help much appreciated, I can't seem to find any solutions out there despite a lot of people saying they are suffering from the same problem. Thanks

Tried these?
- Try signing out of your account and back in again.
- Try removing your card and add it back again
- Uninstall your evil jailbreak