Have a read of this NOW linked article below about maximum number of streams and how the dynamic six registerd devices work on your account.
If you are not using a Boost Membership you are restricted to one stream at a time rather than three and swapping between devices could take 10 minutes to avoid the simitanous stream error message pop up on screen.
If you are using a Chromecast device, then cast to the Chromecast first before you hit the play button on the NOW App on your phone or tablet.
You don’t need to delete the devices. You just need to ensure you have exited whatever you were watching on them, not just paused it, closed the app, or switched the device off while it was playing.
And nor do you need to exit the Now app on those devices; just make sure you are back at the Home Screen level.
We have been talking about it taking ten minutes or so for Now to notice that we have stopped watching on a device, but we now learn that this is apparently only in the pause/close app/device off situation, and if you ensure to exit as above, you can switch to another device immediately.
|You were recently watching on a different device||Our system hasn't yet registered that you've stopped watching on the other device.||When you stop watching on your first device, make sure you exit the show or movie you were watching rather than just pressing pause, turning off the TV or closing the app. This will signal that the stream has ended.|
If you don't do this, you might need to wait up to 10 minutes before you can start watching on another device.
I can’t test this, as I have Boost which gives me up to three devices at once, but maybe you can.