I by accident discover how to get the KF to generate carousel covers as they are sent your Fire! No more clearing Kindle Data needed once you update the covers in Calibre by reconverting or Save to disk. It wont fix covers in white that are already on your KF just all the new books sent from now on.
It's a cover size issue. I don't know why but I recently install a plugin for Calibre called
Resize Covers. By default it resizes the cover to 450x680. I started using this as my default cover size for new books I've added and tonight it dawned on me, all the books I sent to my Fire using this size have shown up on the carousel!
This explains why every now and then a cover would make it through. It was the size!
The ones with the other sizes still show up white but now we know how to fix this problem for future books! *does happy dance*
Someone can experiment with different sizes if they want to understand it better. I know size 450x680 works. I am a happy camper and now I must go to bed.
Update: You can use the save to disk feature without conversion to fix the cover.