Reverse proxy external www site

I have set up my F5 to reverse proxy internal URL’s to external hosts (I.e. gets rewritten to cnn.com but the user sees it as abc.com).

 

It’s pretty simple. A rewrite profile and the external site as the pool. But there are some sites where it won’t work. Is there something that a web server looks for on the inbound request that would cause it Not to work when it’s sourced from the f5?

 

Just can’t figure out why it works for some sites and not others.