Libreswan VPN subnet to subnet connectivity

We are working with a third party who requires VPN connectivity to access their services. We have setup up a site to site VPN connection with the party using Libreswan and the tunnel is up. We can ping the IPs on their subnet from the main VPN server (GW) on which the tunnel was created, however our application server that sits behind our GW within our subnet cannot ping their server. Tcpdump on the GW shows that ping traffic from our application server comes through the gateway and is forwarded to the other side, however replies stop at the (GW) and are not forwarded to the application server.