Suddenly, Audio Echo Test Fails

I've got three Zoom rooms, all with the same hardware configurations that have been running smoothly for several months.
Recently, they will intermittently report they fail the audio echo test.
Client reports echo happens on participants end, attendees do not hear any echo.
I've restarted all hardware, and can get the echo test to pass by bringing the volume down to unreasonably low levels.
I thought I had balanced the levels correctly, but came in today to see two rooms failing the echo test over the weekend.
Are there further troubleshooting steps? Has there been a recent change in audio sensitivity?