Rickross the post owner in a comment is not necessary. If comments are only between you and the post-owner, if nobody is commenting, then an @username to the post owner at the beginning of a comment will be removed.
if only you and the author have commented on the post so far, the @name will be automatically removed from the beginning of the comment, as it adds no value.
Why do some names not appear in the auto-complete box?
The auto-complete box can determine when an @name would not be necessary to trigger a notification and in that case will not include the user in the list.
Additionally, only users who have commented on the post are ever populated into the list. Editors and other users from the post's history will never appear there, even if they are able to be notified by typing manually.
All the things are taken from META.SE
You can see it clearly i just typed two text in the comment box and an auto complete box appear on your Question
Here is my Question and i am trying to add @Keshav but an auto complete box ain't apear.
It's Clear that We can't Add @username on answer of our own questions but we can do this on another one's questions and answers: