For years now, my Spotify app will only sometimes show the Chromecasts on my network in the device list.  My Sonos speakers, on the other hand, are ALWAYS available in the device list. 

My multiple Chromecasts of varying generations, as well as my multiple Google Homes (of every variety) are only accessible some of the time.  I usually need to force close Spotify and reopen it for them to show up. 

Another way for Spotify to realize I have Chromecasts that it should be offering me in the device list is by asking my Google Home to play music and then my app will start doing so and quickly populate the device list.

This has been the case on all of my Android phones as well as on the desktop Spotify.  Really hope there's a solution-- tired of this being the case for so long!

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Google Pixel 3