Possible Duplicate:
How do comment @replies work?
Say, we've got an answer with some comments under it. Sometimes when I want to reply to one of the comments, I can easily refer to the comment author printing @ and starting to enter the author's name, that is autocompleted for me. But sometimes (if only one person gave comments or not...) I'm not suggested with any variant. Why is it so? On what terms does it depend?