AirPlay works on some devices , not others


I recently set up Volumio, version 2, on a Raspbery Pi 3, with an IQAudIO DAC+, with a music library on a USB stick. It works well.

Now I would like to stream from my Apple devices to Volumio (iTunes library, Pandora, etc.) When I go to AirPlay on my iPhone 6, Volumio shows up and it streams fine. Same with my iPad and my girlfriend’s iPad. But on my girlfriend’s iPhone 6 (same OS version as mine) Volumio does not show up in AirPlay, and it doesn’t show up in either of our MacBooks or my iMac.

Why does the Volumio server show up in the AirPlay menu if some of our devices and not in others? Everything is connected to the same WiFii network. I have tried it with the server connected by WiFi and by Ethernet - that doesn’t make any difference. Clearly AirPlay is enabled on Volumio because it works fine on several of the devices. I am really stumped.