[Grefighter]

I found a epub book online and converted it to mobi for my Kindle. Original book showed a penguin as thumbnail picture. Today I just found out how to use Calibre to create book cover from youtube so I did the same thing, download metadata find the cover online then select the one I want as new cover. Then I converted book to mobi again. After I loaded the book to Kindle, the thumbnail picture is gone. Only book name is show. If I open the book, tap "Go to" and select "cover", it shows correct cover. And if I push power button to let the book cover show on the sleep mode, it also shows the correct cover. But not the thumbnail picture. Could anyone tell me what I did wrong? Forgot to mention, I did the same process with a different book, it worked perfectly. TIA