Mark Blevis over at Podcast Lounge has some great tips concerning your feeds in iTunes. He noticed that iTunes wasn't getting updated with new episodes for his podcast.
After some investigation and a few emails to iTunes support, he figured out was going on. If you have a podcast in iTunes and your subscriber base hasn't grown for some time, read Mark's post for some clues.
Recently, I scaled down the number of podcasts I'm subscribed to in iTunes. My criteria for unsubscribing was just to get rid of the ones that didn't have a new episode in 3-months or more.
I didn't have the patience to contact the host or even visit their website to find out what was going on. Instead, I just unsubscribed.
Chances are, if you're a podcast producer, you're losing subscribers in iTunes due to problems in your feed. Double check that new episodes are being updated so you don't lose listeners.