Please see here for more details on how to pair your Pokémon GO Plus + with Pokémon GO. Please see here for more details on how to pair your Pokémon GO Plus + with Pokémon Sleep.
If the issue still persists, one or more of the following may be the cause of the issue.
・ Drained battery
・ Operational error
・ Issue with the application
・ Issue with your device’s operating environment