Are you using Revoke or Deny when you restrict the Registered group?
The admin after a new installation is actually a member of Registered. I am guessing that you set Deny permissions for Registered which would have overridden the permissions granted by the Administrative group. Try using Revoke instead of Deny.
Hmm. I tested this and I am a little confused. Unless I am misunderstanding how things are supposed to work, it appears that inheritance isn't working properly.
Looking at the Node Permissions... if you have a user in two groups, where group1 has an explicit Revoke and group2 has an inherited Allow, then the result is an overall Revoke. Is that normal? Shouldn't this result in an overall Allow? It seems as though an explicit Revoke in the Node Permissions is overriding of an inherited Allow. There is no inheritance. If I change the inherited Allow to an explicit Allow then it works (results in overall Allow).