Wordpress paginates its feeds, offering by default ten at a time.
You can see older episodes by adding to the feed URL a parameter, /?paged=2, /?paged=3, /?paged=4...
It would be really neat if podcast addict checked subsequent pages on wordpress to see if there are other episodes.
In theory I could do it myself with a copy of the same podcast, but that'd be severely un-tidy :)
Wordpress is very widespread, so this is worth your time. And there may be other services that have the same or very similar behaviour.