Q: RSS Feed Stopped Working in iTunes (Invalid Enclosure)

Episodes 23,22 and 18 do not have 'enclosure' tags containing the URLs of your media files, as required by iTunes: thus iTunes does not see the episodes as containing media and does not display them.

If you look at your website, most episodes show two audio players, the second being a Powerpress player; but this second player is absent in the three episodes in question. Evidently this is resulting in the 'enclosure' tags not being created.

The source code for the episodes isn't the same. The three episodes under discussion are missing this section of code:

Looking at the website you can see quite clearly that they are missing the second ebmedded player and links to the file. I can't advise on how this has happened; you may find Blubrry's forum helpful: http://forum.blubrry.com

Since I don't know how this is all working I can't say whether manually adding the code to the feed would work (you will have to be careful to alter only the relevant bits) - it's possible that it might just revert the next time you add an episode. My guess would be that it's to do with Powepress but I can't advise on that. The forum I linked to might have someone who can come up with a solution.

Looks like you're using the main WordPress feed as your source for Feedburner. This isn't the best feed to use for a podcast. Log into your WordPress site and go to PowerPress >> Settings, Advanced Mode >> Feeds >> Podcast Feeds >> Special podcast only feed and copy the URL of that fee into your Feedburner settings. Save the change in Feedburner and go to Troubleshootize >> Resync to sync the new feed with Feedburner. Please report back when you've done that. (disclosure: I work with the Blubrry support team)

I looked at both of the Feedburner feed URLs you've posted in this thread and neither of them appear to be working. If I try to go to your PowerPress feed http://materialdevolution.com/feed/podcast/ in Google Chrome, it automatically forwards to one of the Feedburner URLs. Are you using a Feedburner WordPress plugin that might be forwarding the feeds?

I assume this is your iTunes Store page? - https://itunes.apple.com/podcast/material-devolution-podcast/id982840466

Subscribing does not work as there is no feed at the registered URL:

and of course the Store will not update - currently it is showing a cached version of the feed at the last time it was able to access it.

This is the original Feedburner URL - http://feeds.feedburner.com/materialdevolution/feed - which produces a file not found error (as does the other Feedburner feed, http://feeds.feedburner.com/materialdevolution/feed/podcast/). The original source URL, http://materialdevolution.com/feed/podcast/, does work.

You need either to get the original Feedburner feed working or transfer the Store to using the functioning feed. Assuming you don't have access to the Feedburner feed you can go to https://podcastsconnect.apple.com (sign in if necessary) and click on your podcast. This will bring up an info page with a field containing the current URL of your podcast. Replace this with the new URL and click 'Save'. Then click 'Update' and the page will check that your feed is valid. If there are any errors you should correct them, then click 'Save' and 'Update' again. This will change the Store to using the new feed, but it will probably take 1-2 days to do so. Note that doing this does not change anything for your subscribers (since their iTunes applications read the feed directly), so any existing subscribers will lose contact with your podcast unless they go back to the Store page and re-subscribe.

Obviously the best solution would be to get http://feeds.feedburner.com/materialdevolution/feed working again - I've no idea how you managed to get yourself into this situation.

You don't need Feedburner to podcast. You'd really be best off at this point by dropping Feedburner and just using your PowerPress feed. The PowerPress feed really is all you need. This guide has instructions on how you can permanently migrate from a Feedburner feed to your own PowerPress feed:

http://create.blubrry.com/manual/syndicating-your-podcast-rss-feeds/leaving-feed burner/

As far as the image for your podcast feed, just make sure you have the image set properly in the PowerPress feed. You don't want to rely on your theme to handle this as it may cause problems, especially if you do decide to change themes.