Sometimes a parent or student will make a purchase using a credit card and the order will come back as either Unpaid or Declined. If there are payments that are going through from other purchases and being marked as paid then the issue is most likely with the cardholder, not the system.
You can look up more info about the declined transaction in the Transaction Express Virtual Terminal by going into the Credit Card Transaction Detail Report tab.
Navigation: Transaction Express > Credit Card Reports > Credit Card Transaction Detail Report
Find the transaction that did not go through by entering the appropriate To and From Dates
Click on the Blue Transaction ID Number to pull up the info. The Authorization Response Message is why the purchase was not completed.