Are they connected to your wifi?

From FAQ’s:

Does Move 2 support stereo pairing?

While connected to your home WiFi network, you can use the Sonos app to pair two speakers

Thanks. Yes. They are both on WiFi. 

Odd that they both work when grouped, but not when stereo paired. What is your source of music?

Have you submitted a system diagnostic within 10 minutes of experiencing this problem, and called Sonos Support to discuss it? But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system. I’d be interested in what they say.

The only reason I can think of would be that they are not the same sort of speaker, but that theory breaks down when you’re allowed to stereo paired them. 

Closing this out…

I spoke to support. While paired in Stereo mode and playing, I had to power off and restart the speaker that wasn’t work. Fixed it!