Likewise, having connected to a music library, if I stop Firefly or close itunes, the Soundbridge still thinks they are there and lets me choose them and then says “Connection to server failed”. This means that regardless of if the library is active or not, the Soundbridge is not interacting with it through the Belkin, except when I do a restart.
Discovery works via multicast dns.
When firefly (or itunes) starts, it sends pre-caching announcements to the other mdns servers on the network via multicast. These will look like udp packets from the device to 18.104.22.168 port 5353.
Once it’s up, it doesn’t spam those anymore. Instead it waits for mdns queries (which again look like udp 5353 to 22.214.171.124) from the device asking for servers that have a specific service. The server should respond, and everything is okay.
It looks to me like something isn’t right with multicast on your setup. Like multicast is only going one-way across the link.
You can try double-checking all your devices and seeing if they have an option to enable IGMP or multicast, or maybe even something about media streaming.
Failing that, I don’t know a way to *make* the devices work with multicast (other than a round of firmware updates, potentially).