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.