Certain podcasts not syncing to iPhone when subscribed through iTunes
Are you using an iPhone 3GS? It's possible that the creators of the video podcast changed their video format to high definition. If that is the case,  these items will not sync because the 3GS can't playback certain HD video formats that are targeted for other more powerful Apple devices like iPad or Apple TV. 

More info on supported video formats: http://www.macxdvd.com/mac-dvd-video-converter-how-to/iphone-video-format-supported.htm