There's a support article that indicate those posts in which the total number of tags and categories exceed 15 are automatically banned from appearing in the reader. The support article is here:
With that in mind, I've kept that total number under 15, but I still have posts not included in the reader categories for cats and photography in an on-and-off manner over the past month. And, of course, I have some crossover in which a post is for both cats and photography. It defeats the purpose of tagging and categorizing posts.
However, I wonder if it's connected with the number of changes in the UI over the past 60 days.