Also when you click a topic it disappears from your watchlist until its edited again. Other than that I se no reason why it wouldn't be there.
I still don't get why topics with posts before yesterday won't show up. Maybe there's a 24 hour rule? Or maybe I've read them before? I can't remember.
What do you mean by "edited again"? It will show up again if there's a new post?
You must've seen them all, I can see stuff from today. You could click on a topic, click mark unread and it'll still appear in your watchlist also.
And yeah once a new post is made (or if the last post is edited afaik) it'll reappear on your watchlist (but I'm not sure about the edited part if it's still the forums policy).