It sounds like you have at least three machines
- a TFTP/DHCP server,
- the DHCP client
- a capture machine.
The chances are that the capture machine is not even seeing the network traffic, so I'd think about the network configuration. Your options are
- Connect all machines with a hub rather than a switch.
- Connect the capture machine to a mirror port on a programmable switch.
- Install the capture software on the TFTP/DHCP server.