Have you looked at the default cache settings? How long does it take for new posts to appear? Do new comments take a long time to appear as well?
I just conducted a test where a new post and a new comment didn't appear when I first viewed my blog in my test browsers but a refresh of the page caused the new post and new comment to appear just fine.
So, if you have a busy site, you will want to look at your database and browser cache settings, at the very least, to make sure the current settings will work for you and adjust them as necessary.
Peace...