My original post is here, but it's rather long-winded. Bagginsbill's first posts in this thread describe the problem quite well, but the easiest way to demonstrate it is simply to subscribe to a thread that looks as if it will have plenty of replies (you know the sort, I'm sure...). You'll get an ebot for the first reply, but never any more unless you manually unsubscribe and then re-subscribe; or if you post a reply.

The same thing will happen if you reply to a thread and check the 'Notify me' option. You'll be automatically subscribed but only ever get one ebot.

I would ladle on some dripping scorn of my own, but BB does it much better than I. :)