I have converted some epubs to mobi with default settings.
The ePubs all had cover art showing on the iBooks bookshelf.
When they are on the mobipocket reader's book shelf some have cover art and some don't. They all have cover art when the book is opened and I select cover from the TOC
The only thing I could find on this subject was left unanswered.
https://www.mobileread.com/forums/showthread.php?t=77352
Any suggestions?