@liquidprinter Since this thread is a year old, we need all of your specific details. Fog Version, Linux Distribution and Version, and the model of the target host you’re trying to network boot, if you’re network booting in BIOS mode or UEFI mode, and what boot file you’re using, and what you’re using to hand out the file (DHCP, ProxyDHCP).
Also, you might want to take a look at the WiKi, there are troubleshooting articles in there. Here’s one on TFTP: https://wiki.fogproject.org/wiki/index.php/Troubleshoot_TFTP