Hey mate,
what phone, and pair of headphones are you using? Have you tried it with a different headset?
Are the headphones showing as connected in your settings?
If they are… connect them > settings > Bluetooth > find the device > select the settings cog next to the device and then make sure that settings such as ‘input device’ are toggled to on.
Also make sure, that your headphones aren’t connecting to any other device. And, no other headphones are connected to your phone.
Finally, if needs be, you could always reset Bluetooth & WiFi settings in the hope it fixes it.
