I received a downvote on this question, and added a comment that said "Downvoter, care to comment?" -- a convention I've seen hundreds of times (on SO) attempting to learn from the downvote.
The comment appears to be deleted now. Is it a bug, or did someone delete the comment?
Just to show an example where an author of an answer asks for a reason for a downvote, and subsequent comments indicate disagreement with the downvote and that they provided an upvote, see this answer on SO.