Devices dropping from network

Network Discovery is a pain in my experience.  I don't find it generally reliable after decades of using Windows networking.  Some random server seems to always be missing from the list - random - sometimes one, sometimes several - at the moment, I don't see any, but if I \\servername\ I can get to them.

How it is turned on / off depends on the Windows version - sometimes you'll get a message that it's off, offering to turn it on or to make a network "private"...or here...

I suspect you have a Windows problem since all your devices are connected, just Windows networking not finding networked servers.  In the past when I have seen problems like this, I have Googled and changed SMB protocol settings and whatever else is suggested and sometimes it helps.  But like now, nothing at all. I reckon it's an explorer problem and may be cured if I restart explorer.

Something else you can do is look at the list from a command prompt - run CMD.exe and then type "net view".  That should return a list of servers which is working for me now, just not from explorer.