Modal not diplaying registration page within lightbox

Hi I seem to have an issue with the modal not displaying the registration page content.
At the moment the modal ONLY seems to display the registration content on the following page:

www.mydomain.com/user/register (View attached "register-page.png").

However on any other page, for example on the homepage when I click the register menu link, the modal pops up but does not display the registration page content within it. (View attached "register-page.png").

I'm assuming it's the settings - where should I be checking?

Note: I have created a manual registration page in my main menu with the url path going to: user/register

Any advice would be great.
(I have attached my settings also).