I have replied to a thread that is linked to this item, but for some reason, that reply has not brought the associated thread to the fore

When you are viewing the forum in the index mode, if you click on the Order option it will change between date or last reply order. This means that the at the top of the page the latest thread that was started will display, or the thread with the latest reply will display and they will then run down the page in which ever option you have choosen.