There are a couple of reason why this might be happening...
Episode has not been added to the playlist
Each playlist in Omny Studio generates its own RSS feed. This means that all the audio and metadata associated with a playlist is what will display in iTunes and any other directories that are linked to that playlist’s RSS feed.
When you upload a new episode to Omny Studio, make sure it’s added to the right playlist and thus to the RSS feed. There are a couple of ways you can do this:
Upload directly to the playlist from your computer
Click on the playlist, then click “Upload to Playlist” in the top right-hand corner. You can also drag and drop audio files into the playlist.
You can add as many clips to the playlist as you like and schedule them to publish when you want them to. Do this by setting the clip to “Private” and then scheduling the publishing time in the menu that appears. For more info on how to schedule your podcast episode head here for step-by-step instructions
Set rules to automatically add episodes to the playlist.
This way, if you upload an episode to the Clips section of Omny Studio and it matches a rule you’ve set up, it will automatically add to the right playlist. Set up rules by going into the playlist and clicking “Auto add clips”.
Add clips to a playlist from the clip library or the upload screen.
When you add a clip to the library, you can add it straight to a playlist in the upload screen:
You can also add clips to a playlist after they’ve been uploaded to the clip library by clicking “Add clips from library” in the playlist screen.
Episode is set to private
Clips in Omny Studio have three different visibility settings: Public, Private and Unlisted.
Public means the clip is publicly viewable, indexed and can be shared or embedded on a website. Private means the clip is only able to be viewed by users with access to your Omny Studio account.
Unlisted means the clip is not publicly viewable, but can be shared via the Omny.FM link, embedded in a website, or linked directly to the MP3.
Note: if you add an unlisted clip to a playlist, it will become visible in the playlist if the playlist is shared (as a unlisted or public playlist).
If your clip is set to "Private", it will not appear in Apple Podcasts or other podcast directories. To change a clip's privacy setting, click on the clip and scroll down to the section marked "Publishing".
RSS feed episodes limit
If you have more than 100 episodes in your playlist, older episodes may no longer appear in the RSS feed.
By default, the playlist RSS feeds are limited to showing the most recent 100 episodes to optimize for performance.
Note: the RSS feeds provide additional pages to see older content, most podcast apps like Apple Podcasts do not support pagination so will only show the number of results on the first page.
You can change the number of episodes shown by adjusting the "Podcast RSS episodes per page" under "Advanced settings" in the playlist details page.