Why won't my smartphone connect to my Audi A5's Bluetooth?

I have been trying to connect my smartphone to my Audi A5's bluetooth, but it keeps failing. I have tried restarting both devices and checking for updates, but it still won't connect. In the past, it has worked fine. What could be the problem?

First, make sure your phone is in discoverable mode and try reconnecting to your car's bluetooth. If that doesn't work, try clearing the bluetooth cache on your phone and restart both devices. Sometimes it can also help to delete and re-pair the devices.