The issue described indicates that there are more critical priority access points than the backup Cisco WLC can handle with its available licenses. Since there are only 10 licenses available and 7 access points are set to critical priority, not all of them can failover when the primary Cisco WLC is at full capacity. This oversubscription of critical priority access points prevents all of them from failing over as expected.

References: CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide