Feedburner refuses to process a feed that exceeds a certain size. When we put all the episodes in the feed, it gets too large and the entire feed drops out of iTunes.
We keep hitting the limit on feed size. Every couple of weeks, we remove 10 podcasts from the start of the feed.
The tech support guys at Libsyn and Feedburner defend this by saying that a feed is supposed to be what's new, not an archive of everything.