If you want to know see what posts have been made since your last visit, simply go to the forum index, that is the forum's top level, and you will see that a number of options appear in text at the top right of the page.
One of these is "View posts since last visit".
The option ONLY exists in the forum index, so if you think it has disappeared that will be because you are not in the forum index.
Also this option is fabulous for seeing the most recent posts too, so you can see where the most recent and active chat is without having to search for it.
How do I find out the new posts since my last visit?
Moderator: The Global Mods