Mouseover preview of forum threads from the main forum page?

I can't tell if I am supposed to be able to view a preview of the OP of forum threads from the main forum page, when I mouseover the thread title. I get a preview from maybe 1 out of every 1000 threads (I'll go months without seeing a mouseover preview):

Can anyone explain if I should be seeing this for every thread, or if it's only in certain scenarios that I would get one, or if the whole thing is just bugged out?

Thanks!

Most Polls have mouseover previews, although some occasionally don't work.

@mb: They don't seem to ever work for me. When they do, it's a big surprise.

I've never noticed that most of them don't work, when I see them I just assumed they all did that. You've ruined it for me now.