If the application ports on the EC2 instance is not whitelisted for the traffic from loadbalancer, the routing will not happens which inturn causes failure of requests.
This article explains the steps to allow traffic from loadbalancer to EC2 instance.
Step1: Create a Security Group for Load Balancer, say "LB-SG".
Step2: Create a Security Group for instances, say "App-SG".
Step3: In App-SG, permit inbound traffic on the desired port from LB-SG.
Every instance associated with App-SG will permit inbound traffic that is coming from the Load Balancer.