in xdmcp session Unlock popup password not appeair

Ununtu 12.04LTS (fresh install)
-------
config xdmcp
/etc/init/lightdm.conf

[XDMCPServer]
enabled=true
-----------

from admin computer

When I connect to the ubuntu-computer with xdmcp

$ X -query ubuntu-computer :1

When I try to unlock admin access in the "User Accounts"

the password popup not apeair and I cannot add new user with the gui gnome-control-center

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gnome-control-center 1:3.4.2-0ubuntu0.11
ProcVersionSignature: Ubuntu 3.5.0-32.53~precise1-generic 3.5.7.11
Uname: Linux 3.5.0-32-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.3
Architecture: amd64
Date: Tue Jun 11 08:16:09 2013
InstallationMedia: Ubuntu 12.04.2 LTS "Precise Pangolin" - Release amd64 (20130213)
MarkForUpload: True
ProcEnviron:
 LANGUAGE=en_CA:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_CA.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-control-center
UpgradeStatus: No upgrade log present (probably fresh install)
usr_lib_gnome-control-center:
 activity-log-manager-control-center 0.9.4-0ubuntu3.2
 deja-dup 22.0-0ubuntu4
 gnome-bluetooth 3.2.2-0ubuntu5
 indicator-datetime 0.3.94-0ubuntu2