I have discovered that, despite recommendations, it is not advised to use the Wordpress plugin WP-Cache if you use password-protected pages as it stops them working correctly. If you turn on WP-Cache and view a password protected page that you have not previously viewed it will cache a copy of the login page and never let any visitors access the content; if you view a page that you have previously viewed before enabling the cache it will cache the full page, letting everyone who views your site see the page. This is not a good way to work, so I recommend simply not enabling this plugin.