I'm trying to reply to this comment by mentioning the name of the user who gave the answer (using @user syntax). The problem is, that the user name is not appearing at all, at the beginning of my follow up comment. I tried with the first part of the name, and with both names, still no success. What am I doing wrong?
Generally speaking, when a name is not auto-completed, it's the sure sign that the user will be notified. No need to spell out his name. You don't need to remember "quirks", or how Stack Exchange works. Just understand that if a name is not auto-completed, that's because it doesn't need to be.
This is especially useful when you want to address two users at the same time, because Stack Exchange only allows you to mention one with
@. So when it comes to picking one of those users, if one of the names is not auto-completed, then you know to pick the other one.
When you try to use
@<username> on a question, to tag the O.P., it won't work.
But don't worry, the O.P. will get a notification anyway.
All comment without any
@<username> are relatively safe to assume they are to the O.P..
It took me 2 years to understand this quirky behaviour.
The other quirky part is that you can tag him/her in a comment in an answer. But for that to work, the O.P. has to first comment on it.