In iTunes, if you do not listen to a podcast for more than 5 days, the program will stop automatically downloading the podcast for you. Unfortunately, this is a default feature of iTunes, and there is no way to modify it or stop it from happening.
If this is what has happened to you, you will see a small "!" next to the title of the podcast. Clicking on this exclamation point will show the iTunes message, "iTunes has stopped updating this podcast because you have not listened to any episodes recently. Would you like to resume updating this podcast?" Clicking on Yes will prompt iTunes to resume getting future episodes, but you'll also have to hit "Refresh" in the bottom right corner to load any available episodes from the past 7 days. After confirming your username and password, all available episodes should begin automatically loading.
Please note that Apple's servers only allow us to make up to 7 days' worth of episodes available at any given time. If you've been away from your computer for more than 7 days and you'd like to get all missed episodes, you'll have to download them from the show's website and manually load them into iTunes.