There's a couple of settings you probably need to adjust in order to show more episodes in your RSS feed, and in turn, your Apple Podcasts listing.
1.) Log in to your WordPress site. Hover over the Settings menu on the left-hand side and select Reading.
2.) Change the number in the "Syndication feeds show the most recent" box to a higher number like 50 or 100. Click the Save Changes button.
3.) Hover your mouse over the PowerPress link in the left-hand sidebar and select Settings. On the next screen, switch to Advanced Mode if you're not already there
4.) Click the Feeds tab. In the "Show the most recent" box, enter the same number you entered in step two. Click the Save Changes button.