The file names of many of your media files have spaces in them. This is a fatal error: spaces are not allowed in a URL.
Browsers, and evidently Podcasts app when subscribing, get round this by automatically substituting the code %20 so that the URL will work: however the 'Browse' section of the app does not. This means that it sees the URL ending at the first space, and as obviously that doesn't work, it sees no episode media and thus does not display the episode. (The web preview page works because browsers handle the spaces as I described).
This apparently affects 142 episodes, in both the media file URL and the 'GUID' tag. You were originally replacing spaces with a hyphen, but stopped doing this on 9 June 2019 as far as I can see. You will have to amend all these episodes, either by replacing the spaces with hyphens or underscores and then amending the URLs in the feed, or by replacing all the spaces in these URTLs in the feed with the code %20 - that will work.
Additionally:
You have 521 episodes: Podcasts will display only the latest 300 of these (though subscribers on a Mac or PC may see them all.
The video version of the episode of 10 Nov 2019 has the URL of the audio version and so won't show as the GUID tag is duplicated (and there are spaces).
