Suddenly Can't Connect to MOST ARD Clients

Running ARD 3.3 on my MacBook Pro. Accessing a clients network via VPN (to xserve running MacOS X Server 10.5.8. Remotely connect to client's server via VPN and then use ARD to remotely access other servers or workstations. Been working OK for years. Suddenly today the only "devices" that I can access is the server that I'm connecting via VPN. If I perform a new network scan, the only devices that show up in the scan is this one server and a few other devices such as some APC battery backup devices. There should be at least 30 or so devices showing up. I'm not aware of anything we have done to change anything. Since I have access to this one server I can use Network Utility on it and have been able to successfully ping and do lookups on other servers. If I open a Finder window on this server and go to the Shared area in the sidebar I can see all kinds of systems showing up. Why isn't ARD seeing them anymore? Any suggestions on troubleshooting what might be going on?