Minimum requirements for Active Directory Failover to work are described here:
Release : 14.3, 14.4
ADS_Failover and ADS_RETRY will only take place if one of the below errors happens:
LDAP_SERVER_DOWN
LDAP_UNAVAILABLE
LDAP_UNAVAILABLE_CRIT_EXTENSION
LDAP_TIMEOUT
LDAP_LOCAL_ERROR
Other LDAP errors should not cause failover and will need to be investigated.