# How to mention someone in comments if his\her name doesn't appear after typing its first letters?

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?