Connecting Windows machines in a subnet or in another subnet

I'm having very mixed results connecting to Windows computers in our local subnet. Some work and some don't. Firewall settings allow WMI. RPC and WMI services are running. Logins are all the same Administrator account for WMI purposes. Turning the Windows Firewall on the targets off doesn't seem to matter if it's not going to work.

It sometimes appears that using a computername works better than an IP address. Does that make any sense?

I can only use IP addresses for 2 other connected subnets. Some of these seem to work - so the IP address and other subnet must not hurt. Yet, not many of them have connected either.

What's the secret? Shouldn't this be a bit easier?