A company has a VPC with a public subnet and a private subnet. The public subnet contains a web server (EC2 instance) that must be accessible from the internet. The private subnet contains a database server (EC2 instance) that should only be accessible from the web server. The web server's security group allows HTTP (80) and HTTPS (443) from 0.0.0.0/0. The database server's security group allows MySQL (3306) from the web server's security group. However, the web server cannot connect to the database server. The network engineer has verified that the web server can reach the internet and that the database server's security group is correctly configured. What is the most likely cause of the connectivity problem?