allow users to buy new site from frontend

Recently, users where unable to signup for a new prosite because they had already registered their email addresses, previously. I suppose they are meant to login and then upgrade or buy another site?

But that doesn't make sense to them. They want to visit the homepage and signup - even if their old subscription has been cancelled.

Is there any setting I"m overlooking that will allow this? I do have "enable multiple" selected in the prosites settings.