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Rule of thumb (pun intended) for Send-to-Kindle books:
Covers show when the book is downloaded as plain old mobi.
Covers don’t show when the book is downloaded in the newer kf8/azw3 format.
This has to do with a bug in the firmware that prevents the OS from generating thumbnails for KF8 books that come as Personal Documents (this bug has been present since the introduction of the format and will likely never be resolved since Amazon has introduced an even newer format, KFX, that, however, cannot be used for personal documents).
The appearance of covers for Personal documents can be influenced:
If you email a book as mobi, it will always generate a thumbnail cover.
If you email a book as an epub with the extension renamed to .txt, it will never generate a cover (but of course, enable some of the firmware features tied to that format like hyphenation).
The same goes for the Send to Kindle app on Mac and PC.
I know that this has nothing to do with the original problem per se, but could help in troubleshooting the issue (and can be used for further reference). For example, which format are the books that do not show covers? If azw3, your problem may have to do with the format. Since for azw3, covers are pulled from the servers (and the generate cover fallback is faulty), this may have an effect on their appearance on your Kindle.
Usually, it helps to wait. If that doesn’t work, often delete, reboot, reload will help. If that doesn’t work either, the only option is a firmware reset.
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