I haven't been able to narrow down what exactly is causing this. In some comments discussing it within slashdot articles themselves - they seemed to think it had to do with the depth of the comment --- but that is not the case.
Here is a (example) comment that is only 1 level deep, and there is no Score displayed for it at all.
http://games.slashdot.org/comments.pl?sid=2079200&cid=35789340
View and moderate all "bugs Discussion" comments posted by this user
Mark all as spam, and block user from posting to "Bugs"
From my observations, it looks like it happens when a nested comment has its parent comments hidden. If you display the parent posts, the moderation score will appear.
Last edit: Anonymous 2013-07-01