[Bermudagirl]

I have just recently bought a Kobo Touch and am managing my books through Calibre.
I had no problems with the first book I read, but I have just started another book and I am having problems with the book cover.
The cover art displays fine on the home page, and also in the collection to which it has been allocated.
But unlike the first book which displayed it's cover art when I put the ebook reader to sleep, it only shows what seems to be a default white cover with the title of the book and underneath the format (epub)
I really liked the proper cover art showing when I wasn't using the Touch, so I really would like to sort this.
Has anyone else come across this, and if so, have they come across a way to rectify this problem?
Any help would be much appreciated!