F5 BigIP LTM Cookie persistence not working !

Hi guys. I have a weird behavior here. Using LTM with SharePoint 2013 and a VS with cookie persistence. when I load the webpage I see resources (images) being loaded from 2 web servers. I expect the session to be persisted to one server for the same url. Any insight on this. I know cookie persistence is not required for SharePoint 2013 since the introduction of the distributed cache service but we need this persistence for our custom dev.