Classic ELB Cross-Zone Load Balancing Disabled

An Amazon Classic Elastic Load Balancer (ELB) is not configured to balance traffic across availability zones. Classic ELB distribute traffic to multiple targets hosting your application. Availability Zones are multiple, isolated locations within each AWS Region, they are the first line of resiliency within AWS. If your application is configured to use only one availability zone it's possible that your application would not be available if that zone has an issue. Configure the Classic ELB to use multiple availability zones.