Selector not working?

Selector not working?

I have made an userscript for forum. ( http://userscripts.org/scripts/show/72031)
It works fine in  http://www.discuss.com.hk/forumdisplay.php?fid=40
but not in http://www.hk-pub.com/forum/forum-1-1.html

$("span") works but $("span a") returned 
[] when i debug in Firebug.

What's the problem?