At this point, to get more qualified help, you might consider telling us a bit more about your environment:

OS + version (of the DHCP server)?
DHCP server software used + version?

Again, where the configuration files can be found depends on the implementation, so don't expect more than generic suggestions as long as you do not provide more specific information.

In general, a daemon will re-read its configuration only if made to do so. The common way of doing so is to send signal 1 (i.e. kill -1 <PID-of-dhcpd>) but maybe your implementation does it differently - again: tell us what you use and we may offer better/less generic help.

If you hoped for something like press the foo-key three times, turn right, spit over your left shoulder and all is good i have to disappoint you. ;-)) I am, at this point, at the same loss you are and by suggesting whatever comes to my mind as remotely possible i hope to help you to get the right idea eventually.

It is possible to enter rules into the configuration so that "MAC-address X" is always given "IP-address Y (plus other specific information in the DHCP option fields: default route, DNS server, ...)". It is also possible to configure the DHCP server to only answer requests from clients explicitly defined in that way - which implies ignoring all the others. So, provided you have such a setup (and maybe you are not aware of that) and if you did not enter explicit configuration for the new servers it might lead to what you saw. If you have configs for the new systems, this point is also moot.

Another possibility crossed my mind: if you have a firewall between you and the client it might block some of the traffic. This is rather unlikely after what you said as you come across pretty sure the server itself doesn't answer but again: i try to come up with ideas, you have to test them.

You are welcome. If - once you found the solution - tell us what it was and how you resolved to problem we all have learned something and we have to in fact thank you for making us wiser.

I hope this helps.

bakunin