Sometimes (especially with fairly new members) I can't get their @mention name to show up. I suggest putting the @ mention name on the profile page. (I can get to that and could look it up.) I ask?
The reason new members can be hard to find with the @mentions feature is that folks are ordered by Karma, so newer people (with less Karma) will show up much further down the list.
A quick solve: the @mention name doesn't appear on the profile page anywhere, but it does appear in the URL of that member's profile page!
Yours, for example, is jimh:
If you just type in whatever appears after the users/ ( aka the user "slug") it will help find the person much faster.