I had a ton of strange podcast app behavior recently. Days and days of podcasts not downloading, no matter what I did. (Though they would PLAY if I pressed play, via cellular or wifi.) "Preparing to download..." never actually made the jump to "Downloading."
I also had a rash of old podcasts being set to download as "new" even though I had played them months ago. Some were even more than a year old.
I did have TWO podcast episodes that kept giving the "This podcast episode is not available for download" error and would not play no matter what.
Here's the other thing: I couldn't get any of the normal app updates in the app store to download successfully, either. I have apps set to update automatically, and when I looked today, there were 15 apps that were waiting to download (several days worth) and even if I pressed Update Now, nothing happened.
It was like there was bottleneck where nothing would download, except, of course, all of my other apps were working, I was getting email, I could use the mobile web, etc, etc.
I tried to close and open the podcast app. I cleared all downloads and tried to select the downloads I wanted. I re-set the phone. I tried messing with the "download over wifi" and "sync with iTunes" settings, I turned wifi off and on, reset my router, (to see if it was a cellular vs wifi issue, in part). Still nothing. Podcasts and app updates were all still frozen.
Finally, I cleared/canceled the pending podcast downloads. I closed all apps. (I even closed iTunes on my desktop, just in case there was some weird "sync" or backup issue going on.) I shut down the phone, let it sit a bit. Powered back up. Opened App Store first, and updated all of the pending app updates. (They started ticking off in rapid succession and were completed quickly.) I opened the Podcast app and started to download all 14 un-played episodes, one by one. I did not queue them up, I waited for each one to download before initiating the next one. I now have all of the podcast episodes downloaded and ready to listen to, and all apps are up to date. I have absolutely no idea if this answer will help anyone else, but I am hopeful that my bottleneck issue has been resolved.