Try this solution
Manually pair the secondary earbud
1 Go to the Apple Watch Bluetooth settings and check which earbud is connected to the Apple Watch
2 Press the button on the earbud not connected to the Apple Watch for 3 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.
Are there other wireless electronic devices nearby?
Is there any object in the line of sight between the primary earbud and the device?
Andriod screenshot.
Apple 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.
Perform a factory reset of your earbuds
1 Place both headphones in the charging case.
2 Press and hold both buttons until the earphone LED's flash white.
3 Wait for the LED on each Melomania 1+ to blink blue three times.
4 The LED will then go back to the “breathing” white display.
5 At this point, you can release the buttons as the reset procedure is complete.
6 The earphones will no longer remember any of the devices that have previously paired with them.
7 When removing either 'Melomania 1+ L' or 'Melomania 1+ R' after a reset, you will hear the voice prompt saying “Waiting to Pair”..