My issue is identical to this topic: https://www.googlenestcommunity.com/t5/Speakers-and-Displays/Google-Assistant-transfer-command-new-p...
NOTE: It involves a radio station (in my case BBC R2).
The station plays fine on any speaker where I request it. However I can't tranfer / cast / move it to any other speaker.
This issue does exist with 'Music' media such as Spotify or Youtube Music
That certainly is not the experience we want you to have ― let's check it out. By the way, thank you for the clarification that this concerns is only happening when playing radio. We might need to isolate this further because other music platforms are working but not the radio. What Nest speakers do you have there? Have you tried other radio stations? It would help a lot if you could do a sequential reboot first. Unplug the power cord of your router followed by your Nest speakers. Plug them back in after 3 minutes tops. Restarting your phone might also help.
Let's try moving the media using your Google Home app:
Check out this guide for more information.
Keep us posted.