GoKawiilCarPlay is happily running Maps and playing music, and then suddenly, Siri stops listening. Maybe it does hear you but won't answer. Or it responds in a subdued tone, and you begin to think the voice assistant has made the decision to mumble for the duration of the drive.
The thing is, Siri itself may not be the problem. A failure can begin with the settings on the iPhone or the CarPlay connection or the car's infotainment system, and finding out which one is misbehaving can save you from changing a dozen settings that were just fine to begin with.
You can activate Siri in CarPlay by holding the car's voice-command button or pressing and holding the CarPlay Home button on the display. Some cars also allow you to use "Hey Siri." If the button wakes Siri but the wake phrase does not, it may be that your car simply doesn't support voice activation.