I have a nexus 3 hub with centerlock disc mount
shimano rotor
I mounted the rotor to the hub and the hub/internet wheel build to the bike myself so error would not be overly surprising.

When I lock the rear brake I can push the bike back and forward about one inch before it locks out and skids the tire.

The rotor is not moving in the pads and I'm 99% sure the rotor is not moving on the mount.
That leaves some apparent slop between the outer hub where the mount is and the actual axle.

Is this just the nature of this combination?
If so, fine, I can easily live with it.
There is only an occasional minor 'clunk' when braking,
I'm only concerned if there is an error or defect that will get worse.

thanks