This KB Article refers specifically to iTunes
Q: I'm updating my podcast, but iTunes isn't showing any of the recent episodes.
A: iTunes should update your listing at least once every 24 hours. If you made changes more than 24 hours ago that are not yet reflected in your iTunes listing, there is a good chance that your feed has broken. When iTunes encounters a broken feed, it ignores the feed and continues to display the old data.
A set of recommended steps to fix your feed:
1. Ensure your podcast's feed is supported by iTunes and that you have not added in any unsupported tags.
2. Use a feed validation service like feedvalidator.org to check for particular problems. This may be helpful in determining a specific problem with XML syntax.
3. If you used a software or online-based feed creation service, check to ensure your settings there are correct and iTunes compliant.
4. Subscribe to the feed in iTunes to see if it works. Open iTunes, go to the Advanced menu, choose “Subscribe to podcast ...” and enter your feed’s URL.