I will say that perhaps the reason the author couldn't get into those authors is not that they lost the ability to read longer material, but that they have some

I will say that perhaps the reason the author couldn't get into those authors is not that they lost the ability to read longer material, but that they have somehow matured or evolved to a point where a certain type of book that used to be entertaining just isn't anymore. And that's ok. At some point, you've tapped a book, TV series, or other similar content for all you're going to get. It starts to get formulaic, and it's fine to move on.