Updated my Sonos S1 app this morning, and now I can't group any more than two speakers. If I try to add a third, the new addition plays, but playback immediately stops from one of the speakers that was previously in the group. Seems to be random as to which one stops playing, and it'll still play individually.
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Try a refresh of your network, including the Sonos speakers. Unplug all Sonos devices from power, then reboot your router. When the router comes back up, plug the Sonos devices back in, wait a couple of minutes, and test.
That seems to have sorted it.
Thanks Bruce 
That being the case, I’d consider looking in to how to set up reserved IP addresses in your router, to keep this issue from recurring, although this same process will certainly work again, if your router gets ‘lost’ in the IP table again.
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