iTunes sometimes gets "stuck" for lack of a better term. What happens is there is a temporary error in a podcast feed. It tries 3 or 4 times and then it gets put on the "don't check this too often" list. As long as your feed is up, working and not too big (less then 512k in size), itunes will ping it sooner or later. I would say that 2 weeks to a month is not uncommon.
There is nothing you can do other then go to itunes and your listing... Then click on "report a concern" and click on "out of date". In one case it took over 2 weeks of doing that every day before itunes updated the listing.
Remember, your subscribers are getting your newest episodes even though the itunes app on their computer.