The auto complete form doesn't seem to like some users' names on Hydrogen Audio, such as one user, whose username is ff_mfg, but their display name, and thus the necessary mention name, is "FF / MFG", and the forum does not respond to "@FF / MFG ".
E: I see I could just use the 'member' BBC tag, but I don't see an easy way to insert the text for that from the editor, and again, auto complete doesn't work if there's whitespace in the username.
@ff / MG test
@FF / MGF test
@FF / MGF again
Curious.
The mentioning work, but for some reason the "backup" apparently doesn't...
@FF / MFG
Anyway, I feel it's the same reason as http://www.elkarte.net/community/index.php?topic=3873.0