Being puzzled by this - particularly as the episode which hasn't appeared is only a few hours later than the one which has - I've had another look at the feed. There is nothing in the feed itself to cause the problem; but it does occur to me that there is one other possible explanation.
We've had issues in other podcasts occasionally where the iTunes application would read and subscribe to a feed perfectly well, but the Store wouldn't - I've never seen a convincing explanation about this but it looks as if some servers don't react properly to requests coming from the Store.
Dropbox is, to be honest, not really a good place to use for website or podcast hosting. It does work fine with the iTunes application, as we've seen, but I suppose it may be a possibility that it's not handling requests from the Store properly. That would explain the symptoms - I'm assuming that you added the new-feed-url tag at the same time as you added the second episode of December 16th. The Store is looking for the Dropbox feed, because of the redirect in the old feed, but if it's not getting a response then it will continue to display the cached information from the old feed.
The Subscribe button works because although it's sending the application to the old feed, there is an immediate redirection there. (If you subscribe manually to the old feed it immediately changes to the new one).
In the event that my guess is correct, you will have to start by asking Dropbox whether they can advise. If you can't get any satisfaction there, and the Store still doesn't update, you are in a bit of a bind since you can't add another redirection tag to be read by the Store.
You would have to move the feed and the episodes to another server, add the redirection tag to the DropBox feed to redirect the iTunes Application yet again, and then email podcasts 'at' apple.com and ask them to move the Store to the new URL.
As I say I'm running on guesswork here, but if you don't get an update in the Store then it may be your only way to proceed.
