Phone can't detect Bluetooth headphones

[Kit5]

My skullcandy hesh 3 headphones doesn't appear in my A51 phone but it can be connected to other devices like laptops or Tv. Why? Please help

[Penguinbo]

Can you see other Bluetooth devices on the phone?

[Kit5]

I don't have any other bluetooth devices but I can connect to my laptop and my old phone which is s8

[DannyT]

Try this.

Head to Settings > Apps > More (3 dots top right) > Show Systems Apps > Bluetooth > Storage > Clear Cache. 

Then make sure your headphones are in Pairing Mode and try the connection again.