I don't know if this is the problem, but my iTunes sometimes decides to stop updating podcasts because it thinks I am not listening to them. This is because I rarely listen to podcasts on my computer. I transfer them to my iPod and listen to them on there (yes, sorry, I still use an iPod). You would think that iTunes would be able to tell that I have been listening to the podcasts when I sync the iPod, but apparently not.
When this happens, there is a little exclamation mark next to the name of the podcast. If you click on that it gives you the option to resume updating the podcast.