Import from CSV behaving oddly

I am using the import from CSV plugin and populated the sample spreadsheet with a small sample of 8 books that I wanted to add. First time worked a treat but then I decided I wanted to make a small tweak to the name so deleted those books , amended the spreadsheet and tried again.

No luck , I got a message say “0 books imported successfully” and indeed there weren’t. I have not been able to work out why the books won’t now import