Q:"Why is my Bluetooth not working? My iPhone will not recognize any Bluetooth devices. How do I fix this?" – Fred from Apple Discussion
iPhone or iPad users who run iOS 11 or iOS 12, alternatively updates its iOS version to a new version, likely get stuck with Bluetooth issue, such as Bluetooth not showing up on iPhone or iPad, Bluetooth won't turn on, Bluetooth suddenly disconnecting, Bluetooth won’t connect after iOS upgrade, Bluetooth connection always drops and Bluetooth can't recognize Bluetooth devices as Fred reported. You may have been obsessed with this Bluetooth not working problem for a long time and still on the way to look for effective solutions. Now good chance comes. You can follow available ways to fix it soon.
1. Enable/Disable Bluetooth
Note: You can go to "Settings" > "Bluetooth" to enable the switcher if there is no way to totally close Bluetooth as well as WIFI on Control Center (iOS 11/12).
2. Forget All Paired Device and Match Again
3. Restore iDevice or Reset All Settings
Note: Remember to back up iPhone or iPad in advance as this solution may easily wipe current iOS data.