When the selected computer is down or when the Desktop Central agent is not installed in the client computer.
When the domain administrator credentials specified in Desktop Central while defining Scope on Management is no longer valid. If it has been changed, specify the modified credentials by modifying the Scope of Management.
When tried from a browser that do not have java plug-ins.
When ActiveX controls are disabled in the Internet Explorer browser. Enable ActiveX controls as explained here.
Verify whether the IP Address of the selected computer shown is correct.
If you have computers with same name in two different domains/workgroups, specify the IP Address of the computer and connect.
When the firewall running in the machine where Desktop Central Server is installed is blocking the requests. Ensure that you have TCP port 8443 added to the exceptions list.
When the "File and Printer Sharing for Microsoft Networks" is not enabled in the network adapter of the client computers. Click here for the steps to enable.
When the "Remote Assistance" has not be added to the Exception list in the Windows Firewall.