Why does lemvotes.org only work on some posts/comments?

The question is basically the title.

It's very hit-and-miss. But mostly miss. That picture of a cat hiding under wrapping paper taunts me.

It doesn't seem to be related to what instance the content is on. But I haven't been able to find any pattern.

Is there an alternative (for non-instance-admins) that works more reliably?