HomePod 2nd gen stereo pair volume control

• To start make sure all software is up to date on HomePods and Apple tv
• Reboot you wifi modem
• Ensure all three devices are using the same Apple ID.
• Try restarting to see if this resolves the issue. Restart the HomePod pair. Check to see if both have a swirling white light indicating both are restarting. Or you can just unplug each for 10 seconds to restart.

If this is only happening while being used with Apple TV, restart that as well. You can do that in Settings + System - Restart.

• Also make sure the Siri Remote is charged, test if it’s the remote by comparing how volume works while operating the remote compared to calling out to Siri on HomePods.

If none of this resolves the issue you can factory reset both or the troublesome HomePod(s):

You must ungroup the stereo pair before you can reset HomePod:

1. Unplug HomePod or unplug the power adapter for HomePod mini, wait 10 seconds, then plug HomePod back in.
2. Wait 10 seconds, then touch and hold your finger to the top of HomePod.
3. The white spinning light will turn red. Keep your finger down. 
4. Siri will say that your HomePod is about to reset. When you hear three beeps, you can lift your finger.

After you reset HomePod, you can set up HomePod again.

Reset HomePod or HomePod mini - Apple Support