Your API key is not installed and validated.
Check in the Plugin under Podcast Settings Hosting to verify and validate your API key.
Your audio files are uploading to the WordPress Media Library
If your API key is not present or blocked for another reason and you don't see the blue progress bar when uploading your file to Castos, you will instead see a file URL for the podcast that looks like https://yourdomain.com/wp-content/uploads/file-name.mp3. To find the URL for the audio file of your podcast check the File Details in the Episode editing window or in the browser URL window after clicking 'Play in New Window".
You created your new episode by using a Post Duplicator plugin
If you use a plugin to create new podcast episodes like Yoast's Duplicate Posts the episode IDs will not change, writing over the duplicated post in your dashboard. This is the most likely case if your latest episode is there, but not the previous episode. Always create new podcasts by going to Podcast Add New Episode.