iTunes will stop updating a podcast subscription if you have more than five episodes downloaded but not listened to, as detailed in this page:
http://support.apple.com/kb/TA23353
which states:
You've subscribed to a podcast but have more than five unplayed episodes. iTunes will stop automatically downloading newer episodes. You may get the following message:
iTunes has stopped updating this podcast because you have not listened to any episodes recently. Would you like to resume updating this podcast?
You can click Yes to continue downloading additional episodes. Or you can just listen to any part of any episode and a new episode will download at the next update.
There is no setting in iTunes to change this, but there is an AppleScript available which will apparently deal with this (I've not tried it):
http://dougscripts.com/itunes/scripts/ss.php?sp=updateexpiredpodcasts