This is something I’ve had to do twice now, and it’s not all that hard, but I keep forgetting.  The specific problem is when you have a wildcard security certificate for your domain, but your subdomain just points to the same folder as your primary domain – even though it works correctly when you access it’s insecure version.

I’m sure there are other causes and fixes for this, but I find what I need to do is:

1. Log into WHM
2. Search “Install an SSL Certificate on a Domain”
3. Then hit the “Browser certificates”
4. Find the correct wildcard certificate and click the “user certificate” button
5. Under “domain”  change “*.yourdomain.com” to “yoursubdomain.yourdomain.com”
6. Then hit the install button

And that should be all you need to do!  You can just do this with as many sub-domains as you need.  If you want to check what certificates you have installed, you can search for “Manage SSL Hosts” in WHM and it will tell you there.