Hi @fgustafsson83,
I can’t say I know how the Spotify API locates devices to play to through Spotify Connect, but the controller the Sonos app is on must be on the same subnet as your speakers, otherwise it may not see all your devices.
As Bruce suggested, if you have a mesh network, it is likely creating different subnets where the controller and speakers can’t see each other. Since your iPhone could see all your speakers, but your Samsung tablet was missing one, this is likely the case.
This will also be the case if you have any additional networking equipment such as extenders or multiple access points. If you can provide us with information on you network topology, such as makes, models, and how it connects, we can have a better look into what could be causing the issue.
I would also suggest you submit a diagnostics and reply here with the confirmation number. As you have two controllers, one diagnostics from each while the issue is occurring can greatly help in narrowing down the problem, especially if one controller has access to all speakers while the other is missing some.