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Why is it that when I want to mention in a comment or an answer a user, sometimes it displays the name, and other it just doesn't? I'm not sure if that is supposed to happen? Is it because some of them have a space in their user names? Why?
You mean in your comment to Derick's answer? There the message will always go to the author of the answer. So no @reply is necessary. It would become available (afaik) if another user were to comment on the same post.
For the linked duplicate How do comment @replies work?, see a possible answer to your question in item #10: "If only you and the author have been commenting on the post so far, the @name will be automatically removed from the beginning of the comment..."`
@
sign has no special meaning in answers, only in comments, and subject to constraints.