I’m also using the Sonos app on an Apple mobile device. I’m wondering if you may have inadvertently set up two different ‘systems’, for the speakers, and one is showing up while the other isn’t connected, and vice versa. 

Because resets removes ‘all’ data from a system, I’d suggest care here, but what I’d do is unplug all devices except those that are your ‘normal’ system, and contain all your streaming sources and playlists. Then I’d factory reset the other speakers, and re-add them to your system using that ‘add a device’ process. At the end, you should have a single system that shows all your speakers at one time.

Conversely, you might submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.