Are the One SLs paired as a stereo pair? If so, it probably has something to do with that. The solution would be to limit your power outages.

Also, in addition to @GuitarSuperstar comments if only those two Sonos SL are losing sync you might consider plugging them into a surge protector to avoid damage to the speakers. In fact if the power outages are due to your local power grid..surge protecting all your components is probably a good idea.

How long are the power outages? How “clean” is the power restore? By this I mean does the power return once, or does the power return briefly, go out, then return again? 

It is possible that a brief or partial outage can result in an undefined state for the SL’s. I have no specific experience with SL’s and brief outages, but over the years I have had odd issues with computers and such. And, I’ve seen the inverse where one product starts up very rapidly, the network is not yet ready and the quick starter gives up before the network is fully restored.

In some respects network clients behave similar to telling the kids that if there is a major problem, go to a certain safe location and wait for adult supervision.  If a network client feels that it is stranded it will often go to this certain location and wait. If the adult does not realize that there is an issue and go fetch the client, the client will wait, and wait, and wait. A reboot of the client is usually the only way out of this dilemma. In the case of SONOS clients, they will  shout out for adult supervision approximately every half hour. In my lazy start example above, this will probably result in the stranded SONOS client joining the gang. Of course, for the waiting human this half hour is beyond the human’s patience. This also makes the SONOS system seem intermittent if the human simply walked away in disgust, tried the unit the next day, and discovered that the system is then working.

My own approach to power outages is to roam the house and unplug everything. There are two reasons for this. First, there can be some wild transients as power is restored, sometimes more than a surge suppressor can handle. Second, if I wait a while until I’m confident that power is fully restored, I can restart everything in a controlled order.

If you have frequent fraction of a second outages, a UPS (Uninterruptable Power Supply) is helpful. I keep my main computer on a UPS, not in an attempt to run forever, but since a brief outage will cause the computer to reboot and I’ll lose my current work, with the UPS on the case, I can control the shutdown.

Thank you all for your input here.

I cannot control the power outage and the speaker are connected to surge protector.
But the fact that only this pair is not syncing back, is the reason I added that post. 

If all my Sonos Products would not sync after power outage, I would consider it to be something relating to my home/wifi etc.

But the fact that all my Sonos products revert online, leaving this pair out doesn’t look right to me.

Any sense on that?
 

Thank you again

ron

Each device has its own unique sensitivity to power interruptions. I have not timed this sensitivity for SONOS devices. I have casually noticed that some SONOS devices will survive several seconds without power while others shut down quickly. Also, the “outage” may not be a full outage, but might only be a voltage sag. Each device has its own sensitivity to this. One device might work through the sag while another may shut down or go into an undefined state that will require a reboot to clear.

Submit a diagnostic when you experience this lockout and again after the restart. Record the confirmation numbers. Follow up with support.