When a campaign using external dynamic content makes a request for external dynamic content for a contact that returns no content (an HTTP 204 status code), then the contact is skipped whilst sending to the rest of the contacts continues.
Any skipped contacts are then recorded in the 'Email skipped' report.
Any skipped sends are added to your billing costs.
Use the Emails skipped report
This report lists all of the contacts who were skipped and thus didn't receive the campaign because the request for external dynamic content for the contact returned no content.
You can also:
Select Display to change the view of the report