In this case, if you switch from LAN to WiFi on your desktop, you should always see the speaker as a device.
What could be causing this issue to happen only sometimes, is if you're using the speaker with your phone via Connect, or a different device using the same WiFi. In this case, the speaker will show as an option on your desktop app since it's currently on use.
All the best.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp