This issue reminds me of a spanning tree issue where the target computer gets an IP address for iPXE but then in the FOS engine it can’t pick up an IP address. This is because spanning tree take 27 seconds to start forwarding data, since the FOS engine is so fast it has already given up by the time the network switch starts forwarding data. Wayne’s recommendation to use a mini (dumb) switch will typically improve your chances of getting an IP address since the building switch port never winks (momentarily turns off and back on) net network link as the FOS kernel takes over the management of the nic adapter. Most enterprise switches have a fast start mode for spanning tree called (portfast, fast-stp, rstp, and a few other names) that is typically turned on for access switch ports.
The other possibility here is that the network adapter is not supported by the FOS engine, in this case the nic will never initialize. I quickly looked through the thread but I didn’t see any mention of the target computer you are tying to boot. What device is this? Is it a physical (on-board) nic or a usb network adapter?