There are a number of reasons a credit card payment might not go through even though the card used is valid and has the available funds. The most common are someone incorrectly entering the card number, expiration date or CVV code, or not entering the same address the credit card company has on file for the account.
If a member is having trouble making a payment you can see what the problem is by logging into your Authorize.net account (https://account.authorize.net) and clicking on Transaction Search. On that screen you can enter the member's name to see any payments they have attempted to make, both successful and declined.
NOTE: The name is the name on their credit card, which is not necessarily the name on the account (though 95% of the time it is). To look up the transaction you need to know at least the last name they used.
The Transaction Search Result screen will look like this:
In this example you can see that the customer attempted to submit payment three times and each time the transaction was declined. Click on the Transaction ID and you'll see the full details of the transaction including the reason it was declined.
In this case the card was declined by the issuer and there wasn't further information provided. That could be due to insufficient funds; a security check (maybe this got flagged as a suspicious transaction); etc. If the reason was an invalid number, for example, that would be indicated here.
In most cases Member Splash is able to provide the specific error, such as an invalid expiration date. In this case it would only be able to provide the information shown here.