GTM failing and sticking to a site

I havew 2 datacenters with a GTM frontending the url's and LTM's front ending the apps. The GTM-GTM and LTM statuses work fine. What we want to happen is always hand out site1 until an app fails, then hand out site 2 and stay there until we decide to fail back to site1.

 

 

The manual resume should be helping with that but it's not. I am sure Ii am missing something but I can't put my finger on it.