What's your firmware version?
See this illuminating, wonderful thread:
https://www.mobileread.com/forums/sho...d.php?t=262285
It appears Amazon may have changed something in an extraordinarily stupid manner.
@quiris'
ExtractCoverThumbs app is capable of scanning your Kindle, extracting thumbnails for ALL your books, and placing them in the appropriate location.
If you use it AFTER the Kindle processes your books, it won't get "helpfully"
replaced deleted by Amazon, which is what the problem appears to be (based on @blaenk's similar problem in the above linked thread)
@Vampyr -- I believe KindleButler is designed to do the initial upload, and indeed operates much like calibre, albeit with far fewer features. As such, it is precisely as useless as calibre.