The reasons Siri may not work on CarPlay

If Siri suddenly remembers how to do its job as soon as the phone leaves CarPlay, the connection becomes highly suspect. For wired and wireless CarPlay, the potential connection problems are a little different. If you're using wired CarPlay, Apple suggests using a different USB cable or a compatible USB port. Wireless CarPlay requires both Bluetooth and Wi-Fi, with Auto-Join turned on for the CarPlay network, so either connection can cause odd behavior even if the CarPlay interface appears to be working normally. Apple's troubleshooting list includes restarting the iPhone and the vehicle, forgetting the car under Settings General CarPlay and pairing it again.