If the WiNG APs are running v22.214.171.124 or higher and prerequisite tool is successful, some 3rd party firewalls could be the culprit. The following is what was observed/performed to resolve the issue where WiNG APs would adopt to the Cloud, but un-adopt after some time (anywhere from 1 to 3 days):
Firewall integrator saw that the WiNG APs were trying to connect to an IP address not the Cloud URL. Since this was the case, Palo Alto Firewalls content filtering automatically puts it into the category "unknown" bucket, and the site blocks traffic to unknown addresses. Once the 2 IP addresses (captured from wired trace from AP) were added to the firewall, the APs connected automatically and have stayed online.