That's easy. The very first one - Moonlight Haunting. I've grown a lot as an author since I wrote that book back in 2008. I've learned quite a few things along the way, and I know if I were to write that same story today, it would be a lot smoother and flow better.
Yes! The first thing I try is changing the story I'm working on. I keep a folder of partially written books, and if one stops speaking to me, I try another one. If that doesn't work, then I take about one to three days and read or do something else fun. Sometimes your brain just needs a mini-vacation.
Take the time to unwind! Like with any other job, if you focus 100% of your time on writing, you're going to eventually burn out. Make sure you stop to enjoy life a little along the way.