My reader arrived this morning

Had no problem at all with set-up or connecting to my home wi-fi. But have just bought a book via Reader Desktop from Waterstones. The cover art was showing as "blank" on both reader desktop and the device. So I opened up Calibre and asked it to fetch the cover, which it did. Cover art is now showing on Reader desktop and Calibre, but not on the PRS-T1- the book is there but the cover is a blank.
I'm sure I read a thread a while back about this, but search as I might I cannot find it.
Can anyone help, please?