Hard resets are a lifesaver for that. My Paperwhite got stuck on a book cover once and holding the power button for like 40 seconds did the trick. Sometimes the cache just needs a full clear.
Ever wonder why Kindles get stuck like that in the first place? Read something about the software getting confused if you put it to sleep mid-download. The hard reset is basically forcing it to start fresh. Makes sense it would clear out whatever glitch was hanging things up. Good to know the trick works on the newer models too.