AutoFill’s convenience is undeniable, but there are times when it doesn’t recognise that you’ve saved sign-in details for a form on a site you’re viewing. If this happens in Safari 9, look for a key icon at the right-hand side of the password field.
Clicking the key reveals a menu that gives you the ability to choose from known credentials – not just those you’ve saved for the site you’re using, but also from your entire AutoFill keychain; so, if you use the same credentials for more than one site (a potential security risk, though many of us do it), this enables you to easy enter them on a site whose sign-in form AutoFill works poorly with.
Similarly, Safari sometimes would not recognise the field for entering a new password on a page for entering a new one, and so it would not offer a suggested strong one. In that case, click the key to find an option to generate one.