In the current YouTube iframe (2021), you have to add fullscreen
to the allow
attribute:
<iframe allow="fullscreen;">
If I understand correctly you have an iframe that contains a second iframe (the youtube one).
Try adding the allowfullscreen
attribute to the “parent” iframe.
For full browser support it should look like this:
<iframe src="your_page_url"
allowfullscreen="allowfullscreen"
mozallowfullscreen="mozallowfullscreen"
msallowfullscreen="msallowfullscreen"
oallowfullscreen="oallowfullscreen"
webkitallowfullscreen="webkitallowfullscreen"> </iframe>