As I said, the books that are missing don’t have more than 99 files, it’s always the last few books (after certain point) I add or edit
I use kid3 app on MacOS to edit the tags. I fill both 1st and 2nd tag with just Title (usualy something like “Chapter 01”), Artist (name of the writer, let’s say “Stephen King”), Album (initials and book name, ie “SK - Bag of Bones”), Track number (“01”).
All of the files are tagged the same way. First XX books show without a problem, the last X books I add or edit don’t. That’s it.
The last thing I’m gonna try is to merge the files into one to four files per book, since it seems I cannot add more than like 1500 files, even though I’m following their advices of naming files.
It’s riddiculous - never seen any file limit in any MP3/MP4 player I had! Not to mention I should be able to get much closer to the 8000 files limit this player supposedly has.