In order for platforms like Youtube to ingest your podcast's RSS feed, it needs to be publicly accessible and not set to "Unlisted".
An unlisted podcast series prevents your show from being accessible to search engines and other platforms, except to people with the exact link.
How to Fix
To fix this, go to Podcasts, click Edit on your impacted show, and then scroll down, and ensure that your podcast is "Public" and not "Unlisted".