If you have inserted your embed code into a website builder that wraps a platform specific iframe around your embed code iframe, the fullscreen option may not be available. Here is a specific example of how a site builder called Wix.com disables the fullscreen option.
Wix.com iframe wrapper (note: the “allowfullscreen” parameter is missing)
<iframe width="100%" height="100%" src="http://www-ironbridgecrossfit-com.usrfiles.com/html/914769_e5dbef82nfeieaoj5546f97176dad.html" data-reactid=".0.$SITE_ROOT.$desktop_siteRoot.$PAGES_CONTAINER.1.1.$SITE_PAGES.$c5n4.1.$comp-ixklw2dm.0.0"></iframe>
Stream Embed Code which is nested under the wix iframe (note: the “allowfullscreen” parameter is here), but is overridden because the iframe above is missing that parameter.