Kindle cover not showing after conversion?

Hi there,

I don't own a kindle, but someone who does tells me that the preview of the cover is not showing on her kindle.
The cover seems to be there and I can see it in the editor. The Epub version is also fine on an e-reader.
Is there something special I need to do for a Kindle and the azw3 format? The file was generated from the Epub.

I looked under Tool/add cover and there are two images there.... a larger image and a smaller one (preview size?).

Any advice?