What am I missing with the WordPress lab?

I’ve run through the "First RDS Intance" lab multiple times and can’t get WordPress to install. I’ve confirmed the default group has an inbound rule from web-dmz. I’ve even set up additional EC2 instances thinking I went wrong there but they all "look" right. I just get a WP error that it can’t find the db, bad credentials, etc. Not sure what else I can look at.