Books appearing on one Kindle, but not another?

I have two Kindle Oases, a 9th Gen and a 10th Gen. They have the same firmware, they were ordered with the same options and are virtually identical except for the screen. I copied a bunch of books over using Calibre, one that I got from a Kickstarter campaign and a bunch more purchased from a small publisher. All the books show up just fine on the 10th Gen Oasis, no problems at all. But random books won't show up at all on the 9th Gen unit. I've deleted and reuploaded them several times and I can see them if I open the Kindle as a disk drive, but they never appear anywhere on the Kindle. All the books are in the MOBI format, and both Kindles are the same firmware, 5.14.2, which should be the latest available.

What would cause the books to work fine on one device and never show up on the other?