Can't reply to certain posts/post on certain topics?

I've been looking at my old topics, some of them will let me reply to someone/add another post, others won't allow anything. I would understand if it was all old topics as to not bring back topics from the dead. However some of them will let me reply/post. I'm even noticing this on even newer topics that aren't even on mine. Is there some rhyme/reason that certain topics I can't post/reply on?

My apologies if this is the wrong area.