If you can track this down to some header that I'm not setting correctly, I'd love to hear about it. Otherwise, I can't be of much help. Almost everything I know about NNTP/internet "news"/usenet I learned for this project, and I'm still learning bits.

I've seen this behivior in three cases. One is where the replies are young enough to be on the server, but the root node they go with is not. Not much I can do about this, I have to make a cutoff point somewhere. Note that sn, my news server, whole threads always expire at once, so this will become less of an issue with time.

One is where the root node changes sections between when it is originaly put on the gw and when replies are put on, in some cases. Nodes are never moved, or otherwise changed, after being initialy created. The reason for this is a mix of the nature of usenet, implementation details on my end, and implementation details of perlmonks, but it's unlikely to change anytime soon. This rarely splits threads in half, though, because of caching of the node XML views.

The final, and most common reason, seems to be an annoying limitation in mozilla-thunderbird. When the article's title changes other then a 'Re: ' being prepended, it will break off the thread, and start a new one. Many newsreaders (or at least mailreaders) seem to do this optionaly, but I don't see an off-switch for it in thunderbird. This is a client problem, not a server one, as can be seen by the fact that the other replies are threaded properly, so I can't do much about it.