Chapter 11 IPsec VPN for FortiOS 5.0 : Monitoring and troubleshooting : VPN troubleshooting tips : Check your routing
  
Check your routing
If routing is not properly configured with an entry for the remote end of the VPN tunnel, traffic will not flow properly. You may need static routes on both ends of the tunnel. If routing is the problem, the proposal will likely setup properly but no traffic will flow.