OK, makes sense.
Concentrator has 2 interfaces that you would normally use: private interface which connects to your internal network, and public interface which connects to your FW. From the topology diagram, I assume that your Concentrator is behind the FW, not paralel to your FW. Need to know if Concentrator public interface is assigned public ip address OR if your FW is NATing traffic towards the Concentrator. There are a number of variables to think about, and depending on which one it is, the traffic flow will be different.
Also, for normal internet traffic, how is it being routed?
Not sure if this is possible within your environment, but to test if the VPN Concentrator is working or not, it is probably easier to plug a laptop to the gateway switch, and configure the laptop ip address to be in the same subnet as the VPN Concentrator public interface subnet.
Another option would be if you are from internal network, to connect to the VPN Concentrator private interface ip address instead.
Just trying to understand what is the common issue that you are experiencing with the VPN Concentrator? Just want to know because if you are connecting from the internal network, that probably does not simulate the real issue, so would like to know what is the common issue you are facing with this particular VPN Concentrator.