Why isn't the Iframe app displaying any content?
This behaviour might be encountered due to the fact that the web page that had been added as the Iframe URL has an HTTP header (X-Frame-options) set to SAMEORIGIN for all server responses.
If you wanted to add a website to the Iframe app, but that web page has theHTTP header set to SAMEORIGIN, you would need to ask the server provide if they could amend the X-Frame-options from their side.
You can check if the URL of a web page can be added against the iFrame app using this tool.
More information about the Zendesk Iframe app can be found here.