I'm afraid I can't be of much help here, not having an iOS device to check this on. It all works perfectly well in the iTunes Store and when subscribing on a Mac. Apparently iPhones work from the Store page rather than subscribing, so as the episode appears there I can't see why the Podcasts App should fail to see it.
What may be an issue is that your server apparently does not support 'byte range requests' - this is where only part of the file is requested at a time as opposed to the whole thing: it's the method used by iPhones, and if the server doesn't support it there tend to be problems with actually playing the episode: however I don't see that this should affect the episode actually appearing in the first place.
All I can suggest is that you ask in the iPhone Forum, where someone who has the Podcasts can possibly check to see whether the problem arises for them.