In my 2019 A4, Apple Car Play worked fine, so did Android Auto (Android hardly used).

Then a few weeks ago, Car Play does not connect.  I removed all the stored phones and factory reset the "Audi Smartphone" bit in Settings.  Plugging in the iPhone (iOS 16) gives prompt on MMI screen to activate Car Play, so it sees it, but then does no more.  If you go to Connection Manager, it sees iPhone in the Smartphone option, but it's not ticked.  If you manually tick it, the tick appears then disappears.  I tried a different iPhone with iOS 18, and changed the cable, same problem.

I plugged the Android in (not been used for a year), that did the same thing, tick appears then disappears, and Android screen showed something like there had been an error, device did not meet security protocols.  Something like that.  I forced the tick again, then Android Auto worked fine.

But iPhone refuses to work - it won't stay ticked, it looks like a handshake fails.  Phone does not react in  anyway when connected.  

Bluetooth connection works fine for music/phone etc.

Anyone seen before?

Not sure if you've solved your issue, I am having a similar problem where iphone 12 pro was fine but the 16 pro isn't. The work phone which is some kind of new samsung is fine? 

I changed the cable to a better one, with higher data transfer capabilities and it's partially fixed? It's still having the odd moment

Might be worth trying a different phone, so at least you can eliminate that as the problem. Or get a diagnostic check to see if any fault codes flag up 

My CarPlay still isn’t working, if anyone else has had same.  
 

My iPhone works fine in other Audis

No iPhone will connect to car play in this car.

The quality of cable does matter. Try a factory reset on the MMI system to get rid of any stored phones data.