Most often, this is caused by network issues, between your controller and the speakers. I’d certainly check the wifi interference FAQ, as well as performing a Sonos network refresh, by unplugging your Sonos devices from power (or powering them off, if battery connected), and rebooting your router. Once the router has recovered, power back on your Sonos devices. This will give them both a fresh IP address, as well as a refresh of the Sonos OS. Finally, check your system for updates, in case there are any that need to be applied. 

Experiencing this as well. Seemed to have started with the latest iteration of the app. Very frustrating to deal with.  Tried rebooting  router and speakers…no luck yet. Spotify app can find the main Sonos speaker so not sure why Sonos app  can’t.   

If you powered down all Sonos then rebooted the router and controller, then powered up the Sonos it is unlikely to be the IP issue, if you didn't please try that.

If you did that then you should call Sonos support or start a topic to seek community help. 

Note that the order of ‘rebooting’ in my process is important. 

It's called poor software engineering. And or hardware engineering too. 

But probably most likely poor product management. Because I have been a Sonos customer for over 10 years and I can tell you this... Sonos used to be fast blazingly fast. It was amazing. It was like magic. But not anymore. It's sad to see it's so slow now when it used to be so fast.

I has used Sonos for many years, and has been very satisfied. It has been quick and very intuitiv and smothe to use.

But after there lavest app update, its very slow and app always start by displaying No system found, for a few seconds.

Its so dissapointing how Sonos treat its loyal custommers