Are old podcast episodes removed from the iTunes store?
If the podcast's feed has history archived in the Wayback Machine (which many are), you can use Backfeed1 to generate a feed which includes all current and often2, all historical episodes.




1. disclaimer: I created Backfeed, since I was annoyed at the same problem.
2. limited by what is available in the wayback machine within a sane number of API requests.