Have you set them up as Default Audio?
Set up home theater audio with HomePod and Apple TV 4K - Apple Support
Do a hard reset of the faulty one, you'll need to ungroup the stereo pair first. Use the Pressing the top method: Reset HomePod - Apple Support.
You will need to set HomePod up again on a device that is using the latest software. Stereo pair them again.
Then Restart your Apple TV and set up as home theatre mode . Make sure it’s also running the latest tvOS.
Or you can first just try restarting Apple TV, in case this is why.