Ensuring SSL 100% Secure on Checkout / payment page?

Hi,
on the checkout page of my site, (when user logged in) the HTTPS padlock is appearing yellow and states that not all content is secure. How can i ensure that all resources especially on the checkout pages are completely secure?

Your connection to *URL REMOVED* is encrypted with obsolete cryptography. However, this page includes other resources which are not secure. These resources can be viewed by others while in transit and can be modified by an attacker to change the look of the page.

*URL REMOVED*

When on this page Site showing completely secure - *URL REMOVED*

Support Access open
Test user: *INFORMATION REMOVED*

Thanks

Tom