An obvious one, that you might have thought about yourself; big and small letters is important with this.

@Rebbel won’t work.
@rebbel will.
You have to be precise.

Also it won’t work on the name of somebody who’s not already in the thread.

It seems to work only when the @ + name is the first thing in the message or paragraph, and otherwise at whim. It doesn’t work within the markers for whisper text unless you add the markers last.

And if the list of users in the thread is very long, not all names will come up in the pick list when you enter the @.

The @ will only yield the 10 most recent posters in a thread.

If you want to address an earlier poster, type @ and the first letter of their username.
This should at least yield all usernames starting with that letter, but no others.

If you use the mobile site (https://i.fluther.com/), there is no @ option, i.e., it doesn’t work.

Some people have a name that may not be visibly clear what all the characters in their name are, too, but if you type @, then you can pick them from the list that appears.