DIAGNOSIS: The built-in far-field microphone on the TV is likely disabled. There's a physical switch for privacy, and if it's off, the TV can't listen for the wake word, even if the software setting is on.
FIX: First, find the microphone switch. Look for a small physical slider button on the bottom edge of the TV, near the center, just below the Sony logo. It might be on the backside of that lower bezel. Slide it to the ON position (you should see a green indicator or feel it click). Then, on the TV, go to Settings > Device Preferences > Google Assistant > Allow hands-free and make sure it's toggled on.
IF THAT DIDN'T WORK: Do a full reboot of the Google Assistant system. Go to Settings > Apps > See all apps > Show system apps. Find Google Play services and Google (the app). For each one, select it, then choose Force stop, and then Clear cache. Do not clear data. Restart the TV from the system menu. This resets the voice services without deleting your account.
HEADS UP: The 'Always On' feature only works when the TV is in standby (orange LED light), not when it's fully powered off (white light). Also, if you have any USB devices plugged in, sometimes they can interfere with the microphone array—try unplugging them temporarily as a test.