The iWEB MS payment screens have been designed to be as basic as possible and therefore help the customer solve their own problem should they have a problem making a payment. However if your customer has attempted multiple times and still is not able to make their payment then they would likely be looking for some sort of explanation.
Login to iWEB MS as an admin and go to the Transaction Diagnostics report (click on "Reports" at the top). Make sure that you're looking at the correct day (the report defaults to the current day) and you'll be able to see the customer attempts. If your customer is receiving a payment error then you will see those attempts showing up in red on this report. Below each failed attempt will list the response/reason that was received back from your payment gateway. Remember that iWEB MS is not a direct payment processor but rather uses the payment gateway that you selected. You may see a number of different reasons why a payment does not succeed ... here are the most common examples:
1: "Payment Declined" - This is the most common and simply means that the customer's credit card issuer declined the authorization. A decline is exactly that, it usually means the customer is either over their credit limit or some other reason that would cause the issuing bank to decline the authorization. A payment decline is different from a payment error. Only the issuing bank truly knows the reason for the decline and often that is not included in the response from the credit card gateway so you may stuck only knowing that it was declined and not knowing exactly why.
2: "Invalid Data ... Zip" - Also very common, and may not be limited to just Zip code. What this means is that the customer is filling out the payment form with address information that is NOT on file with their credit card issuer. For example, a customer is making a payment on 125 Main Street, Anytown, TX, 35353 but their credit card statement address is 863 Rose Lane, Anytown, TX, 35543. There's your red flag, the customer may be making a payment on 125 Main Street but on the payment form they would need to provide the address as it's listed on the credit card statement, this this case 863 Rose Lane.
Using the Transaction Diagnostics report allows you to see the errors that the customer is generating and answer the question of why the customer's payment is not going through.