Google Cast devices don't appear unless I cast from another device first

[GravyAnecdote]

My Cast Devices don't appear in Desktop unless I cast to them from a different device first. 

Example: I open Spotify Desktop app in Windows. Only my SONOS devices are visible: 

There's no sign of my Google Home devices! So I say to Google Home "Play x on Spotify". And it starts playing on my Google Home.

Instantly, the desktop app updates showing that it's playing. That device is now visible in the Desktop app. Here's the screenshot from Windows:

 

But I still don't see the other devices in this list. 

When I look at my phone, all my Google devices are visible (this is a screenshot from the phone):

All my devices are on the same wifi. I can cast to all of them via voice or via cellphone. I cannot cast to them via Windows app.  Help!

Plan

Premium

Country

UK

Operating System

Windows