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Verify that the other device's Bluetooth feature is enabled and functional
In order for a Bluetooth communication to be established, the Bluetooth feature should be enabled on both the devices. If it is enabled on both devices, but a communication isn't established and if you verified that your device's Bluetooth feature works, check if the other device's Bluetooth feature works, too. For example, try pairing the device to another device.
Note: If you determined that the other device's bluetooth doesn't work, restarting the device should make it work again.
Make sure that the devices's Bluetooth versions are compatible
Maybe your Bluetooth devices are not compatible. For example a device which has the Bluetooth 2 version won't be able to recognize a Bluetooth device which has the Bluetooth Smart version (commony found in gadgets such as heart-rate monitor, fitness bands, etc.).
Usually, Bluetooth devices are backward compatible, so, you shouldn't have a problem to pair a Bluetooth v2 device with Bluetooth v4 device.
Note: Consult the device's manual to check how compatible your device is with other devices.
If your Wi-Fi router is near the Bluetooth device(s) and you have issues with establishing a good Wi-Fi connection between them, move the devices away from the router. Wi-Fi routers might interfere with the Bluetooth signal as they work on the same frequency.
Make sure to be as far as possible from Wi-Fi routers if you want a strong Bluetooth connection.
Make sure to check for and apply updates on a regular basis because they might contain fixes for bluetooth-related issues, too. Usually, you should receive a notification when an update is available, however, sometimes a notification might not appear.