How do comment @replies work?
Simple enough question. Only once in a blue moon, when commenting, am I able to use the @ button to tag someone. Usually, @ does nothing. Is there a reason for this?
You can only Tag someone else who has already commented on the Question / Answer. Also you cannot @tag the asker when commenting on the question.
The reason for the first restriction is to stop you tagging questions with "Paging @JonSkeet" and notifying Jon to answer your question. The second is because the asker is always notified of comments to their question, so there is no need to tag them.