The trouble with this - as with many other series - is that publishers push the author to keep on writing WAY past the natural end. Which is why the books get boring. The best writers stop while the readers are still begging for more, and go on to write something different.
Can you imagine being a writer and desperately wanting to dump a scenario so you can do something else, but your publisher only wants what you have always done before - and you need the cash?!
Not many writers can produce 4+ novels in a series and still be brilliant.
------------------
Smile! Life is too short for bitching!
Fljotsdale