Siri not working with AirPods.

I have seen this question asked before, but have not found a working solution yet.

I am getting no response when I say "Hey Siri" while wearing my AirPods. I can still talk to Siri if I hold down the side-button on the phone, so I know the microphone is still working.

I have already tried resetting the AirPods, restarting the phone, and switching Siri off and on in settings.

Device info: iPhone 15, iOS 18.5 - I think the AirPods are 2nd gen if I remember right.