Chrome prevents itself from going full screen when any extension is manipulating the sound so you can always see the blue rectangle icon in the tab bar (to be aware of audio is being manipulated). There's no way to bypass it. After all it's for your own safety and that's a good thing.
However you can improve the situation a bit by pressing
F11 (on Windows) or
Ctrl+Cmd+F (on Mac OS) which removes Windows/Mac OS taskbar at least. To get fully working full screen you need to utilize keyboard shortcut:
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Type
chrome://extensions/shortcuts
in the address bar and set your own shortcut for Volume Master.
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Open your video, put it to full screen and activate Volume Master with the shortcut.
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If you exit the full screen, you need to reload the page and repeat the previous step though to make the full screen working again.