The forum has been receiving an incredibly large amount of spam lately; notifications about replies that aren't there will be because the post has been deleted for being spam.
The antispam software the forum is currently using has an issue where it doesn't leave the "last post in this thread" time set to the right thing after deleting spam, so the order of threads is a little chaotic after each spam cleanup.
We're working on fixing this, please bear with us; but in the meantime, don't worry that you are "missing" posts - things display weirdly sometimes but the only missing posts are deleted spam.