It's probably a Computer Browser problem. Microsoft machines can see each other if they are on the same LAN or VLAN, because they generally find each other through broadcasts. If you want to see computers on other IP subnets, then you need to provide your workstations and servers with ways to find each other because broadcasting won't work across subnets.

That's why Microsoft came up with Windows Internet Name Service (WINS) servers and they also use Domain Name System (DNS) servers to act as centralized name-to-IP address listing services. Or you can create text files HOSTS (for general IP networking) and LMHOSTS (for Microsoft networking) to resolve names to IP addresses locally.

Your WINS or DNS servers may be down or the services may be stopped; or your machine is not configured to use the ones for your network; or the (domain) master browser on your subnet is not working properly.

Less likely, but possible, is access control lists between VLANs or LANs that are blocking TCP and/or UDP ports 135, 137, 138, or 139. These are vital to Microsoft networking. Check for these on Layer 3 switches or routers between you and the remote server(s).

If you can ping the IP address on another subnet, but you can't ping the machine by name or map drives to it by looking in your Network Neighborhood or My Network Places, then it's a name resolution problem. In which case you need to look into the Microsoft side further.

Hope this helps.