Option 1.
When you setup your Merchant Facilities, did you get this card type included? It is normal for Visa/Mastercard to be setup and included, but you will also need a separate Merchant Facility for American Express, Diners Card etc.
If you did get this setup, did you notify payments2us.com that you wish to use this facility and provide the Merchant Info?. If not, please contact support@payments2us.com with the details. Please note, additional Merchant Facilities do attract a once-off/one time setup fee.
Option 2.
Please contact your Merchant Facility provider (bank) and let them know which card types are not working.
AFTER checking with Merchant Facility provider, if have not been able to resolve then please contact support@payments2us.com
Option 3.
If the issuing bank is say an overseas bank, then it could be that particular bank that is causing the issue. The card holder will need to contact the bank and ask them to resolve.