title: EC2 instance can't connect to RDS, from same VPC/Subnet
body: <p>I have one EC2 instance that can connect to RDS, and another that can't.</p>
  
  <ul>
  <li>They are in the same VPC/Subnet/AZ</li>
  <li>They have the same IAM role</li>
  <li>They have different (but similar) security groups, both allow all inbound</li>
  <li>The subnets are showing in the configuration for the RDS instance</li>
  <li>The ec2 instance that can't connect just times out</li>
  </ul>
  
  <p>What could be going wrong? Is there any way to work out where the connection is failing?</p>