We've improved the reference system on the forum.
If you post:
If the username reference is too short, for example "@Usern", if anyone has posted in the thread whos username starts with "Usern" they will be referenced so long as they are the only match (and the reference has at least 3 characters in it)
If still no matches are found, it will look for mispellings of users who have posted in the thread you're posting in. For example, if "Username" has posted in the thread, and you make a reference to "@Usneram" they will be notified. This is based on a "similarity" function with a 70% match required.
Also, if posting in someones thread, if not already referenced, the OP will be notified there has been a reply.