I have some issues with no covers on some of my non-Amazon books in my Kindle app on my iPad.
I'm am told that the cover art is to big, but I have tried to resize them ( ~600 x 800 ) to no effect on many.
I have heard that converting mobi to mobi or mobi to azw will fix this, but not having much success with this either. In fact I'm not even sure I am doing it correctly because I can't even find the azw file after Calibre has supposedly finished with it, and as far as mobi to mobi is concerned I can't tell that it's making any difference.
Did I mention that I'm fairly new to Calibre?
I have maxed out my 'Send to Kindle' allotment

, so I have to e-mail the book to my iPad and then open the e-mail and "Copy to Kindle".
Does anyone have any ideas?