My RSS feed is not updating to Apple Podcasts

There are only 15 episodes in https://www.endofthechain.com/podcast-rss/

although there are 20 episodes in https://feeds.blubrry.com/feeds/endofthechain.xml

Since I don't know by what process you are getting your episodes from the blubrry feed to the feed iTunes is using I can't advise on why this isn't happening. The six oldest episodes in the blubrry feed aren't in the endofchain feed, but the latter feed has a recent episode which is not in the blubrry feed.

Neither feed has a 'generator' tag, though I suppose the blubrry feed was created in their process or in Wordpress? As to the other feed, some methods limit the number of episodes in the feed, removing older ones as newer ones are added once the limit is reached, but that doesn't explain why the episode of 2 December isn't in the blubrry feed.

This suggests that you are creating the feeds independently, and that a) you haven't added the Dec 2 episode to the blubrry feed, and b) the endofchain feed is created using a method with a limit of 15 episodes (there is usually a setting somewhere to increase this).

iTunes etc can only read the one feed URL. In your case older episodes have been archived to a separate URL and this is not accessible to iTunes. Your only option is to move iTunes to using another, full, feed - presumably the Blubrry feed would be the best choice?

If you are proposing to move to using a different feed please read this carefully before proceeding:

http://wilmut.uk/move

The endofchain feed was updated today but the latest episode is dated 2 December ('Elastic Supply Money - Brandon Iles, Co-Founder of Ampleforth')

.The Apple feed has updated to that - this episode appears when you subscribe, and it's in the Store. There is no more recent episode in either feed.

I don't know anything about Ghost, but it's likely that it has the limit on episodes I mentioned: look for a setting to change this.