A company is hosting an application on Amazon EC2 instances behind a Network Load Balancer (NLB). A solutions architect added EC2 instances in a second Availability Zone to improve the availability of the application. The solutions architect added the instances to the NLB target group.

The company's operations team notices that traffic is being routed only to the instances in the first Availability Zone.

What is the MOST operationally efficient solution to resolve this issue?