Not really enough information to be sure, but I’d try two things. First, check for updates and apply them. There appear to have been situations recently where speakers running different firmware have had difficulty in grouping properly.

Second, if that doesn’t work, I’d try a reboot / power cycle of the speakers, which will force them to reload the Sonos OS, as well as request new connections to your router.

Bonus answer, if neither of the above works, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call 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.

Greatly appreciate!

Answer provided is sufficient.

Seems like in my case, music was started on one speaker from a device with a streaming account that was casting to a specific speaker.  Trying to add another speaker to that "cast" through the Sonos app was prevented for some reason, probably by design.  In this case, the cast was Amazon music, and in my Sonos app, it complained my Amazon music subscription expired, which isn't true, but whatever.  Buggy shit. Couldn't add another speaker on my existing system to a speaker playing a stream started from another source outside the Sonos app, which to me, defeats the whole point of a Sonos system. Stay off my speakers with casting if I can't play the same source on additional speakers later. 

Edit: seems to only be the older play:1 speaker on Wi-Fi that cannot participate in a multicast group of speakers all streaming the same content.  It can stream the same channel, just not in multicast, so it's forever out of sync. Trying to add it to the other speakers playing gives the unable to connect, try again later message. Seems to be a legacy speaker issue since it's on the same wifi network as all the other speakers, era, 2 x sonos one, Sonos one sl, etc. frustrated still

It turns out, my play:1 was disconnected from the network for a while and may have missed a firmware update or two.  Factory defaulting and re-adopting allowed the app to update the firmware on the speaker and now it can simultaneously stream again with other speakers...