Like milk, podcasts will spoil if left unused for a period period of time -- at least in the mind of iTunes. iTunes will automatically unsubscribe from a podcast that hasn't been listened to within the last 5 updates, and more than 5 days have passed since an episode was played.
The rationale, as Apple states in the documentation for podcast providers, is to "minimize unnecessary bandwidth costs for both the user and the podcaster," as well as making it more likely that episode downloads align with the actual plays of the episode.
But I like the option of being current on all my podcasts, as well as being selective about which episode(s) I want to listen to. And being the rebel that I am -- using a two button mouse in 2001 while everyone else was still control-clicking -- this means that I will sometimes not listen to a podcast within these set parameters.
In the past, iTunes would keep me in check by forcing me to manually click on the exclamation mark next to each podcast to resume updating the podcast. Needless to say, this became a chore after the 10th click. You can also select all your podcasts, right-click and choose "Update Podcast" to force a global update, but why do three steps when AppleScript is there to reduce those to one?
iTunes will keep me in check no longer, as Doug Adams (of Doug's Scripts fame) provides a solution for this issue with his Update Expired Podcasts v2.0 AppleScript -- which batch updates each of your podcasts. You could even set up an iCal alarm that runs the script every day to force a fresh set of podcasts.
Update Expired Podcasts v2.0 (download link) is donationware, and is available for download at Doug's Applescripts for iTunes site (link). It's finger clickin' good.