This also affects in in Ubuntu 14.04, I followed the suggestions in comment #13.
I replace the line "start on runlevel [2345]" in /etc/init/tftpd-hpa.conf with "start on (filesystem and net-device-up IFACE!=lo)" and now it starts on boot.
I'm not sure why but I didn't have this problem with Ubuntu 12.04.
This also affects in in Ubuntu 14.04, I followed the suggestions in comment #13.
I replace the line "start on runlevel [2345]" in /etc/init/ tftpd-hpa. conf with "start on (filesystem and net-device-up IFACE!=lo)" and now it starts on boot.
I'm not sure why but I didn't have this problem with Ubuntu 12.04.