First, double-check that you entered all card information correctly, including the CVC security code. Often a simple typo is to blame for a declined card.
If the card information was entered correctly, but the charge was still declined, there are numerous potential causes. The app will give you as much information about the decline as possible - but most declines are generic, so you may not be able to give your customer a clear reason. At this point, you can politely suggest they your customer contact his or her card issuer, ask for more information, and ask for future charges to be accepted.