Is there an additional gate for down-voting comments? On the bestcomments page, certain comments are downvote-able while others are not. Specifically, from my account your comments cannot be downvoted. Is this intended behavior or a bug?

The gate is on the time of the comment's posting. Comments of more than a certain age cannot be downvoted. The age boundary is somewhere within "12 hours" to "1 day", given what I see on the bestcomments page right now.