My podcasts isn't updating on Apple iTunes

At its simplest you need to add the missing episodes to the SoundCloud feed.

If you want to transfer the Store to using the Podomatic feed it gets more complicated. Full details on how to do this at http://wilmut.uk/move

In summary:

1. Add a tag to the SoundCloud feed. This would change both the Store and your subscribers in iTunes seamlessly. However as you don't have direct control of the SoundCloud feed this will depends on whether SoundCloud provide a method for doing this, which seems unlikely.
2. Adding a 301 redirect to the server isn't possible since you don't have control of it.
3. Changing the feed URL in Podcasts Connect will redirect the Store and future subscribers but current subscribers will not be redirected. The only way round that is to add an episode to the SoundCloud feed announcing the change and asking them to resubscribe.

The title of your podcast is in fact 'Closing the Gap with Denise Cooper' and searching on this in the iTunes Store produces no result. All the episodes in the feed appear when subscribing in iTunes, but as I can't access it in the Store I can't proceed. What is the URL of your Store page?

Yes, that's the URL of your Store page. As you say, it has only five episodes. However, it isn't using the feed you linked to:

https://closingthegap.podomatic.com/rss2.xml

but the feed at

http://feeds.soundcloud.com/users/soundcloud:users:158491588/sounds.rss

which has only the five episodes. Basically, you've been updating the wrong feed (or more likely you've managed to get two feeds but you've only been updating one).

The feed I flagged as being used by the Store hasn't changed. You say 'changing the RSS' - do you mean you think you updated it? or are you changing the Store to using the feed you originally linked to? If you are doing that in Podcasts Connect you should be aware that it won't update your subscribers who will be stuck on the SoundCloud feed.