Quite often a new user asks a question which causes several people to comment on it and the OP later posts a comment, but does not ping any users (i.e. by typing
@commentersName) in their comment even though it is definitely a reply to a specific comment above. I would assume that many of them do not know how to ping other users.
My suggestion is that for new users only (could be determined either by reputation or total number of comments posted), when they go to click the Add comment button and their message will not ping anyone, display some sort of warning message explaining that they may want to ping someone above and include basic instructions. Of course, also include the option to post the comment anyway.
Here's a quick example of what I mean:
Here's the HTML in case anyone's interested:
<div class="message message-warning" style="position: absolute; display: block;"><div class="message-inner"><div class="message-text" style="padding-right: 35px;">Hey there! Try typing an <kbd>@</kbd> symbol and then the name of someone who has commented here and they will be notified of this reply. <a href="https://meta.stackexchange.com/questions/43019/how-do-comment-replies-work">(more info)</a> <br> <input type="button" value="Add comment anyway"> <p><input type="checkbox">Do not show me this again</p> </div></div></div>