IF you truly are not getting a DHCP address, this is not a problem Altiris / Symantec can solve. DHCP is completely out of our control.
IF you ARE getting a DHCP address but not reaching the PXE server, that is something we can help with, but only solve part of the time. The first part of reaching the PXE server is a response to the DHCP request which can be made by either the DHCP server (DHCP proxy or "forced mode") OR can be made by the PXE server itself.
In the latter case, you would need to configure IP helpers to forward the DHCP request to the PXE server as well as to the DHCP server.
In the former case, you need to configure DHCP to pretend it's PXE just for the first packet request.
The problem here is that we don't know what is failing. A sniffer trace can help (eg wireshark) to determine where the problem is.
Another thing you can try is another type of computer/hardware. It's always possible that a NIC is having a problem and requires a firmware update where other NIC's work fine. If you test a computer in 1 port and it fails, but test another computer in the same network port and it works, then the problem is in the computer and you'll need to contact the manufacturer most likely.
That's a start. GL.