If you have trouble using fullscreen on YouTube, here are some tips:
If the fullscreen button appears, but isn't responsive, you may have disabled fullscreen permissions for YouTube. Please check your browser's settings and permissions to make sure fullscreen is available on pages where you might use YouTube.
On Firefox, try the following steps:
On Chrome, you can set your browser permissions following the website permission instructions.
Certain browser add-ons and extensions may disable fullscreen mode. Please check your add-ons and extensions and change your settings as needed.
Fullscreen may also be disabled in the following cases: