When the later books in a series aren't as good?

I've recently added a banners to the side of my blog of some of my favorite series and I realized that I loved some of the first books were awesome but I didn't enjoy the later books as much.

So today's post is about series that the later books aren't as good as the first. One series I can think of is the Beka Cooper series by Tamora Pierce- I loved the first book in the series and I couldn't wait to read the second book. When I read the second book I enjoyed it but not as much as the first. Then the third book came out and I couldn't wait to read it because I loved the first book too much. I read the third book and I did not like it at all! The characters were completely off character, the pacing was off, and there was too much insta-love. This was something I did not expect from Tamora Pierce- she's written some of my favorite series but the first book in this series was definitely the best. There have been other series where I've been disappointed in later books but this was the first example I could think of.

What happens when you start a series and the later books aren't as good as the first ones? Are you turned off of the author or the series? Do you finish the series in a sense of loyalty? What series have you read that the later books disappointed you? Leave me a comment and let me know!
hk