Diagnosis
Grouping failure. This is usually due to firmware version mismatch between the two bulbs or one bulb being in an unresponsive state.
How to Fix It
Individually check and update each bulb's firmware to the same version via the Tapo app. Ensure both bulbs are online and responsive. Delete any existing group, then create a new one. Select both bulbs from the main device list during the 'Create Group' process.
If That Didn't Work
One bulb may have a weak signal. Grouping requires stable individual connections. Improve the signal for the non-responding bulb or move it temporarily closer to the router to create the group, then move it back.
Heads Up
The Tapo app groups are for simultaneous control only, not true synchronized effects like color flow across bulbs. For complex scenes, you must set each bulb individually or use a third-party hub.