It seems autocomplete for user names when mentioning another user is currently case-sensitive. Typing dannyh doesn't offer DannyH (WMF) as a suggestion. It should imho.
Just a +1 that this is really frustrating. I recently tried to @ mention Stillwell, and it took me several tries to figure out the right combination of letters to find her. (Mostly because it always takes me a while to remember that the reason I'm not getting matches is because it is unexpectedly case sensitive.)