Endpoint not able to browse over site to site VPN to backup target

Hello,

newbie here with Sophos.  I am looking at a (new) client I have inherited who have their servers being backed up locally and then across a site to site VPN to a secondary location.  There is one server on a different subnet that has never been able to backup to the secondary location, and the backup agent on this server can't see the secondary backup location.  If I try and browse to the SMB share on the backup target from this server it fails to find it.  I have checked the firewall rules and log viewer on the source Sophos, and it is all good, but I am not seeing any trace of traffic at the secondary (remote) site Sophos at the other end of the site to site VPN.  The servers that can backup ok are using port 1 physical, whilst this other server is using a VLAN with a virtual gateway number 1.7.  Both the LAN the bulk of the servers are on and the IP of the single server are advertised on the site to site VPN. 

Any suggestions as to where the problem could be?  I am guessing the server doesn't know the route to the backup target.  I'm not very good as yet with using the Sophos traffic capture feature, so please bear with me.