DoggCatcher will only auto-download the media for new episodes. So DoggCatcher thinks that the episode is new.
Sometimes, feed publishers change the title of the epsidoes. By default, DoggCatcher uses the identifier called GUID (which is provided by the publisher) to determine uniqueness for an episode. So if the publisher changes the GUID for an episode, DoggCatcher will remove the old episode (deleting the media) and add the new episode. In this case the media will be downloaded again.
You can tell this is happening if you have episodes that are marked as done and then after a feed update, they then re-appear new.
Most often this can be fixed by changing the 'episode identifier' feed option for the feed to one of the other values. The GUID option is the best choice to start with, but the one that works depends on the feed.