creating a tab page for a FB Pages app

I have created a FB Pages app and am trying to figure out why my tab html is not displaying.

when I go directly to my example.com/tab url that I have set in the facebook app settings I can see the expected HTML. When I go to my app tab on a sample facebook page I can see the request come for example.com/tab on my server as expected but nothing is displayed in the facebook page tab!

all I have is a simple

<div>hello world!</div>

and it doesn't display in the tab. Any idea why this is happening?

It's possible you don't have a valid SSL cert installed on your server. Or facebook may be caching an old URL. Check the URL it's trying to load in the iFrame to ensure Facebook is using the correct one.