Usually, I'm able to address a comment to someone by writing "@[whoever]" before the text; but in a comment to a recent question (Compact, densely ordered spaces) I wasn't able to do this: when I typed "@[whoever]," it just didn't appear in the comment at all. Am I doing something wrong?
The way comment notification and related auto-completion works precisely is a little subtle. The details can be found in How do comment @replies work?
The main issue in the current case appears to be that since OP is always notified of a comment, an "@OP'sname" is removed upon submission of the comment (and it is not suggested in tab-completion) as long as there is noone else in the conversation. [If there is an additional commenter "@OP'sname" works, though it is irrelevant for the purpose of notification.]
Some further remarks that seem relevant in the context:
one can only comment-notify users involved in the post on which one comments (question and answers are seperate posts), where "involved" means have posted, have commented, have edited [no tab completetion for those], and have closed and are a moderator.
One can typically write "@something" for users other than the above but they are irrelevant for the purpose of notification (and thus are not suggested in tab completetion). The presence of more than one @user the presence of which as a consequence has a notification results in blocking the comment submission.
As said, this is not at all everything that is to be said, but it is meant to address roughly the key-issues in the current situation.