Did you add to existing system or create a new one? This may be a time when a factory reset is worthwhile, then trying again to add it. 

It might be a case that you have perhaps split your Sonos products into two separate Sonos systems on your WiFi network. A quick fix it to perhaps close the mobile Sonos App fully (slide off screen).

Power off the the Sonos Arc. 

Open the Sonos App and let it connect to your One-SL speakers and when they’re working okay now factory reset your Arc. See the instructions here to do that… 

https://support.sonos.com/s/article/1096
 

With it’s status LED flashing green, goto ‘settings/system/add product’ and follow the onscreen instructions to add the Arc to your original Sonos system and when complete, see if you then see all devices in your Sonos App.

Without more detail, there’s going to be a lot of guessing here. Is there a possibility that you set up your new Arc as a new system, rather than using the ‘add a device’ function in your controller?

Ah, I see @Ken_Griffiths is on the case, I’ll back out ;)

Good heavens, and @nik9669a ! I should be more careful about reading all responses ;)

Yep, @nik9669a beat both of us to the party ha ha

I’m to the point I refresh the tab before I start typing a response. But when all three new posts hit in the same minute there is nothing you can do.

Doesn’t happen often; you guys usually get there first!

You seem to have two threads running, @PatLal   The other one here: