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?