Diagnosis
AnHDMI-CEC (Anynet+) handshake issue. The TV sees the soundbar but isn't passing through remote control commands properly.
How to Fix It
Reset the HDMI-CEC connection. Turn everything off. Unplug the HDMI cable from both the TV and soundbar for 30 seconds. Reconnect it firmly. Power on the TV first, then the soundbar. Go to your Samsung TV's Sound Output settings and re-select "HDMI ARC" device.
If That Didn't Work
The specific HDMI port or cable is the culprit. Ensure you're using the included HDMI cable and that it's connected to the TV's HDMI-eARC/ARC labeled port. Try a different high-speed HDMI cable.
Heads Up
If you have other CEC devices (like a game console) connected, they can interfere. Try turning them off or disconnecting them to see if volume control returns, then you'll need to manage device power order.