VPN

Hi

For testing purposes, we're trying to configure site-to-site VPN between two computers in our LAN.
We used a soho router with LAN IP (10.11.10.10) for it's WAN address and connected both m0n0wall WAN ports to it with IP 10.11.10.20 and 10.11.10.30).
For each m0n0wall LAN address we configured different LAN subnet (192.168.100.0/24 for first m0n0wall, 172.16.0.0/16 for second). To this LAN subnet we connected two computers each in it's own subnet (192.168.100.0/24 first one, and 172.16.0.0/16 second one).
As far as we know, this is some kind of simulation between two different locations that we want to achieve. All is working perfectly, both computers from different subnets can connecct to internet.
Great, then we  configured ipsec site-to-site VPN on both m0n0walls (following step-by-step guide so I'm sure that everything is correct) but the IPsec connection between both server is not established. We checked more then ten times all the settings and configuration without success.
Can you please help up, are we doing anything wrong or what? What else can we check so that site-to-site VPN will finally get to work?

Thank you all in advance