Have you added the new Arc to your existing system? If you can’t see your other speakers you may have created a second one. 

If the two Sonos One SLs are in a stereo pair, remove the stereo pair in the app then try adding as surrounds again.

Nik9669a: yes my new arc added to existing system but it was hard so hard to add it, when i add it, i lost my sub too. I deleted it for created it again and that working. 

Sometimes, connecting speakers with an Ethernet cable helps during setup. You should then be ok disconnecting the cable once the speakers are set up.