@JessicaRuth said the @name call out feature is not working but I have a feeling she's doing it on her phone and it sometimes takes a few seconds for the names to populate. Also you need to be sure you're call out their "user name" and not their name. I run into this every time I go to call out Megan and try using @megan, wondering why it's not pulling up. Then I remember it's @skeetle.
I've also found that typing the first 3 characters will return possible results quicker.