Roon Server can lose sight of your remotes for several reasons.
The most common culprit is that one of the two (the server or the remote) has been delegated to a different subnetwork by a router or access point. The help article above provides some steps for resolving that.
Another possibility is that active security is preventing Roon Server from advertising itself on the local network. On a Windows computer, this can occur with the Microsoft Defender firewall. You’ll need to add Roon as an exception.
Lastly, a version mismatch between the server and remotes can prevent them from communicating. Make sure both your server and remotes are updated by installing the latest version.