OK that sounds promising. Moodle can not control what goes on in the iframe, so if the iframe is there and the content is not, its hopefully just the URLs to the VR contents, and nothing to do with Moodle.
If you open your browser console and load the page with the iframe, any 404 (file not found) or javascript errors should be displayed there. With any luck that will lead you to the source of the problem.
(Browser console: https://developers.google.com/web/tools/chrome-devtools/console )