When sharing a question, I notice not everyone is listed on the "completing menu"?

I hope I am asking this clearly: you want to “share” a question – you start typing the user-name – it gives you some options – but not everyone – as if there are some who are not “on” the list. And I checked – it isn’t a spelling or punctuation problem with the user-name. I know it’s case sensitive.

Thanks Lisa/Mods.