First things first, Nimble uses an active/passive controller setup. That means that all LUNs are "owned" by whichever controller is active and in the event of a failover (intentional or not) all ownership of the LUNs switches to the other controller.
As for your problem, have you installed the Nimble Connection Manager for VMware on your ESXi hosts? This software installs the correct Path Selection Policy on the host to manage which paths to use, how to best load balance them and also how to handle a path down scenario. You can get the NCM by logging in to Infosight, clicking on the Resources menu in the top right of the window and then opening Software Downloads. You'll see the NCM for VMware listed with all the other platform-specific tools.