Manually pair the secondary earbud
1
Go to the Apple Watch Bluetooth settings and check which earbud is connected to the Apple Watch
2
Hold down on the earbud not connected to the Apple Watch for 8 seconds to enter pairing mode. You should hear the voice prompt "Waiting to pair".
3
Pair with the earbud in pairing mode from the Apple Watch Bluetooth settings.
Is there any object in the line of sight between the primary earbud and the device?
ANDROID SCREENSHOT
IOS SCREENSHOT
You can check which of the earbuds is the primary one by looking at your device's Bluetooth settings. The primary earbud will be the one "Connected" to your device just like the "Melomania L" in the screenshots above.
In case if this doesn't;t helps then could you please check the latest firmware update?