I have just set up a second subdomain site using my super admin password. I was surprised to find it didn't automatically set up as the highest level site (in my network that is an expert site).
I tried going to the Network Admin>Sites>Manage (from the Pro Site column next to the subdomain in question). Here I selected Expert under the "Manually Extend Pro Site Status" area.
This did not change the access from the site.
I also tried Network Admin>Sites>Edit (from hovering over the site name). Couldn't see anything to set there. I did try clicking Enable on an expert level theme I wanted to use. I still could not use that theme within the new site's dashboard.
When I look at the info on the Network Admin>Sites>Manage page, I see indications that the site is expired, even though I just set it up a couple of hours ago.
I want this second site of mine to have access to all the things my core site does -- or at least to the highest level.