In both iTunes 11 and 12, I've had problems with podcasts becoming "stuck" - where the blue circle to the left of the podcast name becomes an exclamation point in a circle and the new episodes are not downloaded. Generally, in one of the list views, right-clicking on the podcast and selecting Refresh Podcast does nothing. In the past, I've always been able to find a view where the Refresh Podcast does work. In iTunes 12.3, I could not do so. Finally, I stumbled on a quick fix. Right-click the podcast and select Unsubscribe, then click again and Subscribe. This finally causes the podcast to refresh and the blue dot returns. I would guess that clicking the Unsubscribe (then Subscribe) button at lower right would do the same, but I haven't tried it.