[stumped]

the best way to get improved help is via a screenshot [ taken with the device' screenshot function ] showing the device location of those mobi books which you" know for a fact" are present.

then another screenshot showing a windows file explorer view of that same device folder, which on usb connection, so that we all see what windows sees.

[ you can attach screenshots easily here, no need for any picture hosting. just click advanced... upload when replying.

once you can prove that the books exist, and have settled whether or not windows can see them, further trouble shooting may help

but another round of " I KNOW FOR A FACT.... OH NO YOU DON'T... "would be counter productive

for clarity: if you now accept the .kfx partial explanation, is this just a mobi issue, or are there any other problem formats gone missing ? e.g. have you tried a test send of any non-drm azw files

perhaps you could also acknowlwedge ( without feeling attacked) that many other users have that same fire tablet, and have calibre, and DON'T have a missing books problem, so its very unlikely to be a " calibre problem".