Failed to get file from tftp-hpa with samba share

[Sam Su]

I have a tftp-hpa server installed under /var/lib/tftpboot. I create a subdirectory and using it to mount a samba share

sam@HostA:/var/lib/tftpboot/eco$ cat /etc/default/tftpd-hpa
# /etc/default/tftpd-hpa

TFTP_USERNAME="ssu"
TFTP_DIRECTORY="/var/lib/tftpboot"
TFTP_ADDRESS="0.0.0.0:69"
TFTP_OPTIONS="--create --secure"

sam@HostA:/var/lib/tftpboot$ ll
total 24
drwxr-xr-x 8 ssu ssu 4096 Jun 1 11:23 ./
drwxr-xr-x 47 root root 4096 Jun 1 11:17 ../
drwxr-xr-x 2 ssu ssu 4096 Jun 1 11:23 config/
drwxr-xr-x 2 ssu ssu 4096 Jun 1 17:06 eco/
drwxr-xr-x 96 ssu ssu 0 May 23 10:17 oak/
drwxr-xr-x 2 ssu ssu 4096 Jun 1 11:23 pkg/
drwxrwxr-x+ 24 ssu ssu 0 Jun 1 02:00 swap/
drwxr-xr-x 2 ssu ssu 4096 Jun 1 11:23 tmp/

In which oak/ is mounted as "sudo mount //x.x.x.x/Images /var/lib/tftpboot/oak/ -o user=ssu -o uid=ssu -o gid=ssu"

If I tftp get file from under oak/, it is always failed, but if I get file from other directory, it will succeed.

This setup is working under Ubuntu 10.04 lts, but do not work under 12.04lts or 14.04lts

Any idea what happened?