You most likely need to add explicit base addresses for the both protocols so WCF knows you want to bind to both. Try adding this to your <service> definition:
<add baseAddress="http://your-hostname-here/" />
<add baseAddress="https://your-hostname-here/" />
Also, make sure you're accessing the service via the machine's WINS/DNS name or you need to add an explicit host header to the web site instance under IIS.