Please make sure that all the render slave machines are pointed to the machine where the dongle is plugged in. To do so, on each render slave navigate to the following location: Start Menu > Programs > Chaos Group > V-Ray for... > Licensing > Administration and then click on "Change V-Ray client license settings". Type the IP address of the server machine (where the dongle is plugged in) into the "License Server" field and be sure that the connection port is set to "30304".

If there is a firewall running, it may be preventing your 3D application from accessing the license server. Try temporarily disabling the firewall or create an exception for the specific IP address of the server and the port number 30304.

Also be sure to make sure that the VRaySpawner is running (it should appear in the Task Manager under the "Services" or "Processes" tab).