[Support request] Registration form problem

I have set up a new paid membership level with a PayPal button and created a registration form for this level with the Form Builder plugin. When I test it, the PayPal button takes my money and I receive the ‘Complete your registration’ email with a link, but I follow the link and the form is faulty, it does not display the Username field. I want the username to be the user’s email address by default and I can’t see how to set this (although it works perfectly in other membership levels I set a long time ago).