It sounds like the problem is with the DHCP service rather than Fog. Your Fog install could be fine, but if there is a problem with DHCP your clients won’t even try to talk to fog.
One thing to try might be sniffing out some of the DHCP traffic on your network to confirm that it looks it should. Wireshark should help there. My hunch from what you say above is that your Citrix setup might have a DHCP service somewhere which is handing out DHCP leases in competition with your FreeBSD box. If that is the case, and your client gets it’s lease from that other DHCP server then it wont work.