I think you can check with these basic setups first, and if it's because of the plugin (which I don't think so cus I receive payments as usual), then you can contact them
1. Check to see if you set up the payment the right way (see knowledge base)
2. Check to see if the Test Mode button is turned on
3. Did you make a test purchase by yourself? Because you should make sure it's the payment problem, not the customer's card or something.