Changing book covers

Using Calibre 2.51


I downloaded 1300 books from my Amazon account. All of them coverted to Mobi or AZW3 versions. I downloaded the metadata for each book plus new covers for many of them.

When I open up the ereader within Calibre, the old covers show up, not the new ones I selected.

Is there something else I need to do?

Sorry if this is an annoying question, but I sure could use some help here.

Thanks,
Susan

Edited to add: when I transfer books to my Kindle 7th gen, the old books cover also transfer, not the ones I changed.