Using the car's microphone in CarPlay

I've got a new car with CarPlay and I'm having a really hard time getting Siri to do anything useful. It seems like Siri will not use the car's microphone for voice commands, which seems silly since the car's mic is close to my face and built for this sort of thing. The phone sits down low in a cubby by my leg. I tested an Android phone and Android Auto was able to use the car's mic and had no problem understanding all my queries, even when the phone was outside the car.

Does anyone know if this is by design, something that depends on the model of the car, or possibly something I can fix in the phone's settings?

Thanks in advance for your help and advice!