Hi @gubbins - can you check if they have notifications for @username disabled? I think it might be causing the issue.
(Account settings > Notifications > Words that Trigger Mentions > Your username mentioned "@username").
We're thinking about getting rid of this setting, since when people send "@username" they expect the person to be notified, and it doesn't really make sense to let people turn it off (they can just turn off desktop/email notifications if they want no notifications).