Posibly a bad email address is subscribed to that post.
They are topics that worked fine with all emails going out to members subscribed to the topic and then sat dormant with no postings so they worked to the subscribed members emails before. Then if a subscribed member tries to post again or a new member tries to post, no emails. The same people on the new posts that send out fine are on the old ones that don't sooooooo
Why would a bad email stop a mailing? The server would just get a bounce or an error on that one while sending. The rest should go through.