There's so-called mixed-mode
content being served here - a mixture of secure (https) and insecure (http) resources. The browser is trying to protect the user from seeing the insecure items, so it's not rendering them. This is expected behavior for a properly-written browser.
For example, on another page in the forums, I noticed that there was an eBay ad that's supposed to have appeared on the page, but didn't because it seems impossible to get the ad served securely (via https as opposed to via http) off of the eBay server.
Since HSTS (HTTP Strict Transport Security) has been implemented for the forums.radioreference.com subdomain, albeit only for one hour, all accesses to that subdomain will be issued by compliant browsers securely (https), regardless of whether the link is coded securely (https) or insecurely (http).
Of course, the site is powered by tin cans and string anyway, so some problems are to be expected.
(Insider reference for those who enjoy reading Web server response headers.)