If you're having issues processing payments within ASAP, one of the first things to do is to check and make sure that your payment gateway is functioning correctly. This support article links to the different payment gateways which each have their own individual status pages:
Having issues with your payment gateway not processing transactions?
If the problem seems to persist while your payment gateway is not having any issues with processing transactions, the first thing to review is to see which error code you may be receiving. Here's a link to a support article that shows each of the payment gateway's response codes to failed payments:
Response codes for declined transactions
If you're still seeing declined messages. The next option would be to ensure your payment gateway configuration is correct. Usually, this requires just re-entering the information. This support article will show you how to perform this action:
Entering your merchant account / gateway information into ASAP
If you've tried all three options and are still having issues with processing payments, please contact our support team directly through ZenDesk in order to receive help with this issue. Leave a detailed response indicating the steps you've taken and which specific invoice or invoices that are having issues.