Audiobooks not sync to iphone from itunes

I am having problems syncing an audiobook to my iphone from itunes. I have just bought a new audiobook but I cannot sync it. I have been syncing perfectly OK for more than two years with other books. The book is ticked in the "books" tab under library. The book is ticked in "sync audiobooks" on the iphone tab and so is the "sync audiobooks" tick box. I have two other books on the iphone that have been synced in the last few weeks but this one refuses to sync. I have recently changed to backing up to icloud. I know this doesn't sync audiobooks and I have to do this by cable from itunes. Has anyone any ideas?