We recommend using our latest javascript embed code to embed your tours on your site. See this article for information on how to embed.
If you are using your own custom iframe code, then fullscreen may not work because you did not include the property allow="fullscreen" as part of your <iframe ... /> code. Ensure that property is included. For more information check out: "My embedded tours are not working"
A note about Google Sites
There is a known limitation with embedding tours on Google Sites pages. This is only affected by Google Sites websites. Google Sites disables fullscreen control for any embedded content, including CloudPano embed codes.