Suppose there is a question of a member $A$. I want to mention this member $A$(so it appears in the comment as "@A"). sometimes,when I type the first letters of name "$A$", The name appears in a rectangle above and I click on the name so the member is mentioned. Many times, When I type the first letters , no thing happens!
What to do in this case? Can I mention him\her manually? How to do this? Suppose that his\her name was "$A\text{ } B$"(there is a space, like in "Maths Lover"). Should I type "@MathsLover" or "@Maths Lover"
Added: When I try to mention the member manually, his name disappear when I post my comment. How to deal with that?
Spaces are removed from the display names for matching purposes. So to match Peter Smith you may use @pet, @peter, @peters, or @petersmith.
Quote from here. $\endgroup$