Can't connect to source ip from remote

I have 4 boxes in my VPN chain A-B~vpn~[C,container]D-E
I’m trying to look back up the tunnel from my remote site D/E.
I can ping B from C but I cannot ping B from D which is the server running container C.
I have added a route in D pointing to C but that doesn’t work either.
Chasing iptable tree’s isn’t my day job so I’m hoping someone can enlighten me as to what I need to do here.