can you read them ok in calibre viewer?
have you tried emailing them to your kindle account to see if they then work
if there's no copyright then you could attach one here for others to test with ?
a possible workaround could be to make simple text file versions and see if the fire tablet will allow you to read those
also, check the relevant amazon community forums, there may be an answer there
- lots of threads ( I have not read all of them) but maybe no definitive solutions:
clear the kindle app cache is worh asa try, so is check for software updates
https://www.amazonforum.com/search?f...e=2&search=DRM