I think I have noticed this over time, but tonight for sure I see an issue ....
When I go to just the "Forun" view, it shows no new status in the "Apple II" forum, but "Apple I" does. When I click into Appl2 II there are 20 new messages spread out across about 6 topics. I tried various waves of refresh and such.
I mostly read Apple II, but occasionally look elsewhere ... but I've have noticed over time that I actually need to click into so of the forums in order to see if they have new posts or not.
I've noticed the same, but I think it's "working as designed":
The new icon just means there are new topics in that forum that you haven't seen yet. I don't think it reflects updates or new posts to topics you've seen already.
Topics that are old, from before your account was created, don't get marked as new.
The way I scan for new posts is, sad to say, just by looking along the right column ("Last Post") for times less than 1 day.
This is correct; I think the intent is that, if you want to follow replies to a thread, you use the "subscribe" function (at the bottom of the first post of every thread). Drupal's forum module isn't the most graceful thing but it's the best way to integrate forum-style threads and traditional articles into one interface.