Hey bobh2825,
Thanks for the reply and trying those steps, we appreciate it!
Since your iPhone is able to change volume with a different Bluetooth device, this would indicate that the stereo isn't responding as expected when using CarPlay.
You didn't mention what happened after following the steps in the article we provided earlier, to forget the device and set up CarPlay again.
If those steps didn't resolve the issue, you may need to reset the car stereo also while the Bluetooth connection has been "forgotten". To see if this is an option and/or you need steps for how to reset the car stereo, check the user guide.
There may also be a firmware update available for the car stereo. Check with your car dealership, or the stereo manufacturer.
Cheers!