When an AP moves from one vWLC to another, it may refuse to join the second vWLC. It occurs when the server hardware fails, or a new instance of vWLCs are created. It is recommended to implement server mirroring scheme at the VMware level such as vMotion or some orchestrator. It is highly recommended to retain a snapshot of the VM instance, one from the mobility domain to which access points have joined previously. Then use the snapshot to start the vWLC instance. Access points then join the vWLC. This method can be also be used for priming access points instead of a physical controller.
https://www.cisco.com/c/en/us/td/docs/wireless/technology/mesh/8-2/b_Virtual_Wireless_LAN_Controller_Deployment_Guide_8-2.html
