Nothing is being hidden
Almost always one of these four, in this order.
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If you just installed or updated it, reload the YouTube tabs you already had open
Chrome only runs an extension on pages opened after it was installed, so a YouTube tab that was already open has no FocusTube in it at all until you reload. The same applies right after the extension updates.
Once it is running you do not need to keep reloading: switching something on or off takes effect on open tabs straight away.
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Check the mode is not Off
Open the popup and look at the Mode tab. On Off, everything on YouTube shows on purpose, no matter what is switched on in Settings. Settings decide what gets hidden; Mode decides when that applies.
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On Timer, check a session is actually running
This one catches people out. Choosing Timer does not start anything by itself — it only means "hide things while a focus session is running". Until you press start, no session is running, so nothing is hidden and the extension looks broken.
Either start a session, or switch the mode to Always if you want your settings to apply all the time.
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On Schedule, check you are inside one of your windows
Same idea. A schedule only applies on the days and times you set, so outside those windows YouTube is deliberately left alone. If it is Saturday and your schedule covers weekdays, nothing will be hidden — that is it working as configured.
If you just want it on permanently and never want to think about this again, set the mode to Always.