Adding new IP into Target group of NLB health-check failing

I have a few target groups for my NLB for ports 80 and 443. It has 2 healthy targets. Trying to add one more target, it gets added successfully to target group for port 443 but health check is failing for port 80. If I create a new target group for port 80 and assign it for NLB port 800, then I can add the same target IP into new target group without issues, but cannot add it to old 80 target group. Tried to change the health-check params between TCP and HTTP but with no difference. How to troubleshoot that? I'm sure the service is running well, can send traffic to the port