WordPress MU

As the heading says,

On my main site, the Dashboard User Default Role, has no drop down to gave users roles. (Has the drop down box but nothing written in it)
So anyone who signs up for a blog has the same rights as me, ie Admin

Here is the funny bit. A test site (That is a new blog, a reader would set up on my site) has got the drop down on the dashboard user role. (Dont forget they got the rights of Admin, by default.)

Can I fix it that the main site has the drop down menu.

Thanks for your help.