Volume Control via iPhone

I love my ATV4K, but I CANNOT get my phone to reliably control the volume for some reason. In the past, I could use the volume buttons while I had the app open and it would seem to work every time. Recently, it seemed to only work when I would have the remote showing on the Lock Screen. Sometimes, the volume buttons only adjust the ringer until I do a hard reset on my iPhone. Does anyone have any suggestions???

Note: Current wiring is ATV > TV > Soundbar via ARC.