Alright, looked into this and found the bug. It was indeed only affecting future published entries. And the next entry would have caused a rebuild of the feed.
I've now ensured that it will also recognize future published entries and update the feed when one of those goes live.
Just a heads up - I've made a further change to how the feeds are handled, adding in some http headers that should ensure the feed readers will cache the page effectively.
There's a few changes involved. I've tested as much as possible and have found this new system very effective. But if you notice any new problems cropping up, do let me know!
Oh, and one change I've made could be noticeable. Part of the cache is fixed at 5 minutes. So if you have just posted an entry, then it could be up to 5 minutes for it to show in the feed. Shouldn't be any longer than that though!
[ Edit: Edited on 08-Dec-2010, at 04:41 by Peter ]