[ossec-list] No diff shown in the alert email

[Macus]

I am using the OSSEC 2.6 to monitoring a symbolic link (ie. $HOME/abc)
to a phy dir (ie. $HOME/abc-v123). The syscheck alert work, but in the
alert email, there is no diff shown for the txt file change. Moreover,
I found there is no image of the files stored in /var/ossec/queue/
diff.
What's the problem? is it because the path is a symbolic link rather
than a phy dir? thanks