The client boots to the pxe boot menu screen as per /var/lib/tftpboot/pxelinux.cfg/default. The timer is counting down. Selecting an option causes the timer to reset (or the menu screen to reload) but nothing happens. What is the next step in the PXE boot process? What is it looking for but not finding?
It would help to know what your PXE menu configuration looks like. In the event that the menu configuration is correct, then you would need to put tftpd into verbose mode to see what it’s trying to do.