I can't really see what the problem might be. What you can do however is to see if you can ping the client otherwise, i.e. without using VPN. That would atleast reduce the problem to reachability at the client's end or at the concentrator end. Another reason might be the presence of a PIX on the remote end... actually this seems to be the most likely reason for this behaviour. If you indeed do have a PIX on the remote end, configure it to allow incoming sessions from the concentrator.