If you don't have any service running that uses UDP port 67 then it means there are no PXE representatives installed in your client node neither in your core server (at least there should be one in the subnet where you are trying to boot from PXE).
This is a netstat output from my PXE epresentative:
My advice: try to uninstall and then redeploy the PXE script again to these client nodes and in case it doesnt fix the issue look for errors in the computer event viewer. Also try to deploy the PXE representative to some different computers to see if these install correctly the LANDESK TFTP and PXE services. It should not be difficult to find the offending culprit.