Firstly, just to clarify the terminology, the podcast is the overall collection of episodes, and you're talking about a missing episode (sorry to be picky, but if we don't clarify this it can lead to confusion).
Your episode of yesterday has as the media file in the 'enclosure' tag (which is where iTunes sees the file) a Flash (.swf) file - iTunes does not handle Flash files, and so it sees the episode has having no playable media. It does not display episodes with no playable media, which is why it's not showing.
Your episode media files must be .mp3, .m4a, .m4v, .mov or .mp4 - other file types won't work.
DropBox is not a good place to host your media files - you aren't likely to get a podcast working using it - but I've no idea how it comes to change file types (if it is). You need to host your media fles on an ordinary web server, which needs to be able to handle byte-range requests (as used by the iPhone, and required by the iTunes Store).