Have a look at mDNS. It’s very often the issue here.

If you use home assistant take a look at music assistant, it solves a lot of these issues for free.

Sadly you still need that POS to intially configure it and the occasional update.

That very much sounds like a network problem.

Are both hosts in the same subnet?

Just as a way to soothe the outrage about this POS of an app: A fair share of the problem can be minimised by using Home Assistant and Music Assistant to control the Sonos devices. This way you very rarely need to actually use the app anymore.

Which is good because it’s indeed shit.