Oh man, am I ever having this problem. iMac running 10.9.4 We can't be alone!!!
I have tons of subscribed podcasts and none are updating since the day I upgraded to iTunes 11.4 (why did I even bother). Many of my podcasts show the "Subscribe" option when I right click on them, but selecting "Subscribe" does nothing - no response. I can't even "Unsubscribe" since it thinks I am not subscribed, but won't let me subscribe and I am not subscribed when I used to be subscribed. How is that for insanity? :-) If I go go the iTunes store, find the podcast and select "Subscribe" nothing. I have only been able to delete the entire podcast and then find it it iTunes and re-subscribe - only then do things work as they should. This is not a good options since some of my podcasts have older episodes no longer on iTunes.
I have been using iTunes since day one. I love it. I have 4000 podcasts and tons of iTunes U content. Sure there have been a few stinkers in updates over the years, but this is awful. Someone needs to be fired and someone else promoted. Why Apple, why can't you test your systems now? Testing and QA are basic IT skills. Then again, I'm sure you are too busy with all your IOS 8 problems to care about iTunes right now. I don't expect this to be fixed for months and wish someone would post clear instructions and files to reinstall 11.3.
Oh, if you want a clear example of a iTunes QA screw up - click on the option under the iTunes menu to "Provide iTunes Feedback" - it goes to a page with a form without a "Submit" button!!!!!! A FORM WITHOUT A SUBMIT BUTTON!!!! You fill it out and then there is nothing you can do with it!!! It also has a pull downs for selecting your iTunes version, but ends at 11.2.1 - it is that out of date. So sad. Here is the page the app itself links to. Hello? Either correct the form or get rid of the link. Then again, this may explain a lack of iTunes feedback! :-)
https://www.apple.com/feedback/itunesapp.html