Yes, there are many reasons why you might not notice a new thread. And just about every thread that gets noticed here gets some sort of response (a big "YAY TEAM!" for all our helpful members).
As I've described before, the most common culpret is the timeout function of this forum software. It's far from perfect, but it is better than just about any other scheme out there. Some tradeoffs had to be made, but overall I believe they were the right ones. It was explained to me that the only way to make it perfect would necessitate running custom software on an Oracle server. Maybe someday we'll have the necessary $50,000+ in donations for that
Anyway, the timeout is 15 minutes. If there's no activity for you in 15 minutes it assumes you've left. This has no effect unless you've opened a thread or used the "View New Posts" function. In both those cases, it will mark everything you've viewed as read, even if you haven't.
Another possible culpret is using "View New Posts", taking awhile to view/read/respond to them, and then clicking "Mark all as read" without checking first to see if anything new has been posted since the last time you checked.
Possible solutions involve not using "View New Posts" (impractical, in my opinion, that's too great a feature to not use), hitting "Reload" before you close the "View New Posts" page (the icons will be black if you've really read them, yellow if you haven't), and the ever popular "read new posts in their own window, leaving the new posts window open until you're finished" (of course, you'll want to hit 'reload' again just to make sure you've gotten them all).
You can also use
this link to see every post made in the past 24 hours whether you've read it or not (you can bookmark this link. I know I do.)
And, the final solution is that there's no solution. As we grow, there will be some point at which it becomes simply impossible to keep up with every single new post. This point will differ for every user, but decisions will have to be made. The only answer here is to periodically browse forum-by-forum to see if anything interesting has passed you by. You can also "subscribe" to particularly interesting threads so that you get an email anytime a new post is made to that thread (you can also unsubscribe via your cp).
So now, by way of illustration:
This post has taken longer than 15 minutes to compose. Should I have used "View New Posts" previous to writing this without actually reading everything, all of them would now be marked as read, even those I meant to read but hadn't gotten to yet.