Why do I only see 'X' episodes when I know there are more available?

Podcast Addict displays the full content of the RSS feed you subscribed to.
If you just subscribed to a podcast and only see the last X episodes, it means that the podcast owner is just sharing those episodes via the RSS feed.

Some famous podcasts are doing this like 'This American Life' which only gives access to the last couple episodes + some random old ones. If you want to listen to older episodes you have to go to the podcast website and stream them from there.

You can easily make sure of that by checking the content of the RSS feed. To do so, long press on the podcast on the app main screen, select 'Copy to clipboard' and finally paste the result in a web browser. You will see the exact number of episodes contained in the RSS feed you subscribed to.