Console Gaming Help!

Okay, it's a bit of a long shot, but I have a Fortigate 600c at home running FortiOS 5.6.  All licenses for Forticare IPS etc have expired.  I have SD-WAN enabled, Virtual IPs configured for my locally hosted websites and server connections. Everything appears to be working fine.  I can connect from any device to the internet.

However, everything is not fine because my flatmate cannot connect to specific game servers from his XBox One.  When I say specific game servers, anything EA, XBox 360 or Activision.  All other game servers he can connect to and use online features.

I have VIPs configured to set Open NAT Type. 

If I swap the Fortigate out for the ISPs stupid little router (uPNP disabled), he is able to connect perfectly fine (though NAT Type is moderate).

I also seem to be getting a lot of "Failed Connection Attempts" being logged in the Fortigate which do appear to relate to these game servers.

Can anyone suggest anything that could cause this scenario?

Thanks