I was using DHCP, TFTP and PXE to install debian on a Soekris 4801. The tftp daemon I was using was ATFTPD. It wouldn’t allow the Soekris to PXE boot until I changed a startup parameter in /etc/default/ataftpd. More here
The other thing that I had to do was create a symbolic link to pxelinux.cfg -> /tftpboot/debian-installer/i386/pxelinux.cfg.serial-9600/ instead of pxelinux.cfg.