Some websites do not allow their content to be embedded in an iFrame. This is a configuration that is set on the server called "X Frame Options". It essentially commands the browser "Hey, this website is not allowed to be embedded externally!".
If you are the owner of the website, I would look into how you can enable that header. Otherwise, with permission, I can forward you a script that may or may not get it to display.
Additionally, with permission from the appropriate third party, you can carry over their assets on to your own server where you have autonomy over server side settings.