Q:

Stack Exchange sometimes fails to @-tab-complete a username

When we comment about others' answers or questions, we may want to notify the other user.  As far as I known, we can type @username_you_want to mention the user you want get in touch with. 
Most times I tried, when I type the first word of a username, the system will give a auto-complete of the user, like this:

However, sometimes the auto-complete did not happen.  In this situation, I type the username handy, but I really don't know whether I notified the user successfully.  Here is an example:

I don’t know why this happens.  Did I make any mistakes which stops the auto-complete function? 

A:

When you comment on a post (question or answer) the poster (asker or answerer) is always pinged automatically.   Therefore there is no assistance needed to include their name in order for them to be alerted to your comment. 
For more about how pinging works see How do comment @replies work?. 
I trust the now deleted comment of @Catija posted to this answer

That said, if two people other than the [poster] comment, the
  [poster]’s username will autocomplete.