Card declines are very common and they can happen for many different reasons. Below are some tips for you to follow in case your customer's credit card is declined:
First, run your customers card one more time to make sure it wasn't a passing error.
If that doesn't work, go to your history and look at the decline reason- these will vary. Some decline reasons vary from "do not honor", "insufficient funds" to "invalid card number".
It's important to make sure you're swiping the card/entering the correct card number at the time of the transaction. For further assistance, your customer might want to check with their bank to make sure everything is okay, then try again.