1. Verify your feed at https://castfeedvalidator.com/ and make sure that your feed is valid. Then iTunes should update the episodes within days.
2. If the feed is valid, but still not all episodes display, it is quite possible that the episode number set in your feed is smaller than the total episode number.
You can change the episode number in the feed in the Settings->Feed> Advanced Feed Settings page in your Podbean account. Then Apple Podcast (iTunes) will update your feed within days.
Please kindly note that Apple Podcasts (iTunes) only supports displaying up to 300 episodes.