Why is this RDS security group not letting my EC2 security group connect?

I have an AWS RDS with this security group:

MySQL/Aurora (3306)  TCP (6) 3306 sg-3sf456

Where sg-3sf456 is the security group surrounding my EC2 cluster.

My EC2 cluster cannot connect to the rds. However if I add a second rule to the RDS security group that is the IP Address of one of my EC2 machines and I try to connect to the RDS from that particular machine, it allows me in?

What am I missing?

Edit: here is the inbound rules for my EC2 cluster:

HTTP (80)    TCP (6)    80    0.0.0.0/0
HTTPS (443)    TCP (6)    443    0.0.0.0/0

Outbound, everything is allowed.