A while back (a couple years?) votes (or at least downvotes) were disabled on per-user "overview", "comments", and "submitted" pages, to make it harder to karmassassinate people. Sometimes there's a bug where you still see the vote buttons but if I remember correctly they don't work.
(You can still vote on a comment after following its permalink.)
The vote buttons work, even to downvote.
What's up with that?
In a discussion thread, I can karma vote on anyone.
But if I select a person to see all their posts, for some people, karma is disabled while looking at them, and for others, it's not disabled. Same thing if I look at their posts under the list of all my posts.