Coverart not showing for high def files

I have thousands of high def music files and most of them show on my Ipad naim client with their cover arts. But a few show only the naim default cover art even though there is cover art in their folders. I have  used the  Naim desktop client to manually load the cover art. It will not work. I am stuck with the Naim default cover. Does anyone know what is going on here?