Bluetooth connected, song "plays" but no audio playing through car

[sd99865]

I have an iPhone 6s (11.3.1), and when I start my car (Mazda CX-9) the bluetooth connects as it should, and phone call audio works as it should, but sometimes, just sometimes, Spotify audio will not come through the car's sound system. Spotify shows that I am connected to the car and shows that the song is playing, but the there is no audio and no song data on the screen as there is when a song IS playing through the car's sound system. Other times everything connects and the music pays just fine. Spotify version is 8.4.52.796. I have tried the following: disconnecting bluetooth on both car and phone and reconnecting, unpaired phone from car and re-paired, uninstalled spotify app on phone and then reinstalled, turned off car for extended period and rebooted phone. Like I said, there doesn't seem to be any pattern but sometimes everything works as it should and sometimes it doesn't. Thanks for any help or experience with this!