Try the forum specific to Android e-readers:
https://www.mobileread.com/forums/fo...play.php?f=225
Although the "Kindle Fire" sub-forum of this one might be of help.
The "Kindle Fire" devices run Android.
Your question does span several possible forums, so you may have to ignore the rule about multiple postings.
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It could be a similar problem to the other, hardware, Kindles -
They do not always detect documents/books passed over USB (by whatever means).
If there is such a thing as a: "Send To Kindle (for Android device)" you might try that.