Files not backed up

Hi,

I am new to using my WD My book essential. When I run a back up I always get a message ‘Files not backed up’ for a couple of files. Does anyone know why certain files can not be backed up? What can I do about it? Also it seems the back up does not finish completely as a prompt shows up to ‘stop backup’. If I click on this is asks whether I want to pause or continue. When I click ‘continue’ nothing actually continues. Any advice?

Thanks