First, double check your payment information for typos and verify that your method of payment is valid.
Also, check that you've entered all required information - it's easy to add an extra card number or assume that some information is not required. For example, did you know that your Maestro card actually has a CVV number, used for internet purchases? You can ask your card provider for this number. The more you know!
If the problem persists, please contact us via the contact form in your app with as many details as possible: a screenshot, the action that triggers the problem, the error message, etc... Our Happiness team will be happy to help.