Thank you for posting. We might require a little more information to take action on this post with any effect, specifically:
The model of Sonos speaker(s) you’re using and whether you experience this issue with all of them
Which Sonos speakers are connected via WiFi and via ethernet, as this will have implications for SonosNET and device discovery
Whether or not these Sonos speakers are visible via another protocol, like Airplay
In the meantime, please try the following steps in order:
First, try simply refreshing (the circular arrow button) the Settings → Audio page to initiate a network discovery cycle in RoonServer
If that doesn’t work, restart RoonServer, followed by the Sonos speakers
If you’re using Sonos speakers with only WiFi, try plugging one Sonos speaker in via ethernet to active SonosNET. See if this makes any difference - depending on your configuration, this may change the way network traffic flows between Sonos speakers.
Enter your router settings page and verify that you’ve followed the troubleshooting steps here: