Troubles Uploading Some Podcast

I set my iTunes to always upload the last unwatched podcast, for all podcasts. But what happens is that, for some podcasts, such as ABC news and global news, Dilbert, the videos are uploaded fine every time. However, some podcasts, such as TedTalks, they are only uploaded sometimes. That means, sometimes there are still unwatched episodes, and they are not uploaded. Some podcasts, such as "weekly wildlife highlights" and "Nasa's Jet Propulsion Lab" are never uploaded.

I've been trying to figure out the pattern, but couldn't. Does anyone know what's going on?

At first I was thinking that only HD videos that cannot be uploaded, but "TedTalks" is always HD, and sometimes it uploads fine.

I'd appreciate any help. Thanks.