The merchant might not accept cards issued in a different country. This is often the case, where many businesses only accept local cards.

If it was an online payment, there might have been a technical issue with the merchant. Try deleting your card details from their website, troubleshooting your browser and making the payment again.

The shop or business you bought from never asked Bilderlings to authorize the payment or in other words Bilderlings doesn't receive the request for the payment.

If your payment was accepted, the terminal where you used your card might have been offline. This usually happens on an airplane or underground. If you spend more money than you have on your balance before the merchant captures this transaction, your balance might go negative. When it happens, you need to top up your balance to keep using Bilderlings.