You can find out this information by running a Transaction Details report with the following additional steps:
  1. When selecting the Report Columns, select Error Code and Client IP under the Valued Transaction section
  2. When Configuring Filters, click Edit next to Transaction Purpose is any of the following
    • Mark Declined
    • Click Save this filter
  3. Continue running report

Once the report has been run, review the Error Code to see why a transaction declined.