Migrate AD DHCP to New Server: Some on old and others on new

We used to have one AD box, AD1. We since added two new AD boxes: AD2 & AD3. The idea was to decommission the old box.

I went through DHCP Manager and set the scope and everything seems fine. Right click on DHCP and selecting "Manage authorized server" shows the new new boxes: AD2 and AD3.

I went to one workstation and did issued ipconfig /release followed by ipconfig /renew. The box releases right away and more importantly renews right away and shows AD2 as the DHCP server.

I then went to another workstation but any attempts to "renew" without AD1 turned on and authorized yields no connection. I even turned the box off and then back on. When connected, it shows AD1 as the DHCP server, obviously.

I did see this quesiton. I went to the old DHCP and verified that the DNS server option in DHCP record points to the new box. I also verified the first option in the forward lookup zone that the new servers show up in the Name Servers and the WINS tabs.

I checked and both workstations have obtain an IP address automatically and DNS servers automatically in the IPV4 properties for the adapter. Going to the Ethernet adapter on the second machine going to the old server and seeing if that has the old server listed rather than obtain an IP address automatically was my first thought.

Why can this second workstation not see the new DHCP server: AD2 and how do I fix the problem? Is the problem something that needs setting on our switch? (just a thought)

UPDATE:

I found a key piece to the puzzle. We have 3 subnets

1. .70.*: Servers & Printers
2. .71.*: Workstations
3. .72.*: Phone

The workstation that works, when AD1 is off, gets an IP address on the 70 subnet, whereas the workstation that fails and requires AD1 has an IP address on the 71 subnet.

I also verified that when AD1 is off and I attempt to request an IP address, Microsoft Packet Sniffer shows that the failing workstation can indeed talk on the network, as I see packets flying by.

I would say the problem is with the switch, and still might be a configuration on our main switch, however both the AD1 and AD2 have IP addresses on the 70 subnet.

JoeQuerty said that a missing DHCP Relay setting in our Brocade ICX 6450-24P smart switch (router) is the problem. A Google search turned up this question, which says the same thing. As stated in my question, the only hesitation is that AD1 works and AD2 fails and both are on the 70 subnet and use the same switch with the current settings.