Update Covers

I know this has been mentioned before, but I can't make any of solutions work.
:( I sent several books to kobo without updating cover (the first page showed up automatically) I removed books, sent them back to kobo with updated covers, didn't work. I removed books, did epub-epub conversion before sending, didn't work. (Interestingly, the new covers show when I'm reading book, but when it's finished it's back to old one).I replaced books manually didn't work.
What am I doing wrong?
Firmware is 2.8.