Sometimes when I try to leave a comment the @name syntax does not work. For example I tried to leave a comment for Ulrike Fischer and I could not find a combination of characters that would work. I assumed I type @U and it will fill up the rest, but that does not work.
You talk about the auto-completion of the user name in comments, right?
Could it be that you wanted to add a comment on one of Ulrike's posts?
The original poster of the post you comment on is always notified and the
@username is even removed by the system if you use one and there were no comments of other people.
Anyway, the actual notification is independent on the auto-completion. You only need to type at least the first three letters of the name, so
@Ulr would be technically enough. However, this always notifies the last user which commented on the same post which name starts with these letters. Note that spaces act as terminators, so
@UlrikeFischer would be the safest and most correct way.