The lack of connectivity is generally either vpn client configuration based, or the firewall on the local pc's that are unable to pass traffic would be the first thing I would check.
generally if a vpn client successfully connects, that means that handshake portion is over, a secure connection has been established (port 51) ... however data is unable to use this tunnel for some reason ergo port 500 is blocked or if that is not the case then the traffic is getting to the far end but not returning via the tunnel, (in this case that is not true, as 2 of the PC's are using the same configuration and most likely the same tunnel on the firewall (remote dialup clients) with traffic returning to them.
so back to basics:
1) check the client vpn configurations
a) make sure that the client is setup to "only connect manually" or has split horizon enabled.
2) check that the firewall has not blocked port 500 on the PC, if you are unable to view the blocked list, then
create an exception rule for ports 51 and ports 500 inbound and outbound.
hope this helps