I don't know if this is a bug or an inbuilt function, but I get occasional issues with comments where I can't use the @namehere format to notify a commentator on a particular question. I can't see a pattern to this. Are there built-in exceptions to using the function or is the site bugging on me?
You can use the @username syntax only for users who have posted something in the comment discussion of that particular post. Or people who have edited the post. The OP is always notified. Mods can use a superping to ping users in chat for example even if they have never visited the room.
If that is not solving the issue then it might be that they are using unusual characters. See for example JDługosz. I recommend just using the autocomplete function in that case. You need to use at least 3 letters for the ping to work.
First off, as you probably know, the @name syntax for pinging someone is only supported in comments. It has no special significance in questions or answers.
Second, as Secespitus wrote, the user you are pinging must have interacted with the post you are commenting on. I don't think I've ever tried pinging someone who has merely edited a post; when I ping someone, it's almost always in response to a comment of theirs. And yes, the owner of a post (the user who originally posted the question or the answer that you are commenting on) is always notified of comments added to the post, regardless of any pinging. Naming the owner of a post in a @name is at best a waste of precious comment characters.
Third, you can only ping one user in a single comment. If you need to notify multiple users, then you need to post one comment for each. If the comment you posted originally genuinely applies to several users, then this can be as simple as
@name see above, but I would generally suggest addressing each user separately. That way, each comment can be flagged and deleted as no longer needed (previously obsolete) when the addressed user has seen it.
If you are having trouble typing someone's username, then copying and pasting it (and removing all whitespace from it) usually helps. Just remember to prepend the @ character.
See also How do comment @replies work? on Meta Stack Exchange.
Mod superpings apply only to chat, not to post comments, and are not available to regular (even high-rep) users.