Small business with 2 Windows 10 and 2 WIndows 7 systems. Also a server running 2012sbs.

From the Windows 10 systems, when we click on Windows Explorer, and then Network, the server sometimes shows up and sometimes doesn't on one system, and NEVER shows up on the other. Both are configured identically, best I can tell. One system shows all systems after a while, the other never shows anything but itself and the other Windows 10 system.

Any ideas what I can check?
Many thanks,