DIAGNOSIS: The microphone on the Google TV remote (the one with the Netflix and Prime buttons) can fail, or the Bluetooth pairing between the remote and the TV's internal dongle can get corrupted.
FIX: First, re-pair the remote. Go to Settings > Remotes & Accessories > Bluetooth. Select "Remote" and choose "Unpair." Then, hold the Back and Home buttons on the remote together for about 5 seconds until the status LED flashes. It should re-pair automatically. Test the voice button. Also, check that the remote's batteries are fresh—low power can cause mic issues.
IF THAT DIDN'T WORK: If re-pairing didn't help, try clearing cache and data for the "Google" app on the TV itself (Settings > Apps > See all apps > Google > Clear cache/data). This resets the voice search backend. After doing this, you'll need to set up voice match again.
HEADS UP: The remote uses Bluetooth Low Energy (BLE), not IR, for voice commands. Make sure there aren't any USB 3.0 devices or dense metal objects directly in the line of sight between the remote and the bottom-center of the TV, as they can interfere with the BLE signal.