alt + Prnt SCrn or only Prnt SCrn are not printing screen

fresh install i386.

on keyboard, alt + Prn Scrn or only Prnt Scrn are not printing screen as expected.

scrot from terminal is working as expected though.

maybe the problem is at lubuntu-rc.xml or at Default Applications for LXsession.

lubuntu-rc.xml says:
 <!-- Take a screenshot of the current window with scrot when Alt+Print are pressed -->
    <keybind key="A-Print">
      <action name="Execute">
        <command>lxsession-default screenshot window</command>
      </action>
    </keybind>

<!-- Launch scrot when Print is pressed -->
    <keybind key="Print">
      <action name="Execute">
        <command>lxsession-default screenshot</command>
      </action>
    </keybind>

on the other hand, Default Applications for LXsession says:
Screenshot manager: scrot

pls, could you try to fix once newcomers will not be able to print screen as expected?

best regards,
ibere

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: openbox 3.5.2-6
ProcVersionSignature: Ubuntu 3.13.0-14.34-generic 3.13.5
Uname: Linux 3.13.0-14-generic i686
ApportVersion: 2.13.2-0ubuntu5
Architecture: i386
CurrentDesktop: LXDE
Date: Sun Mar 2 00:24:43 2014
InstallationDate: Installed on 2014-03-01 (0 days ago)
InstallationMedia: Lubuntu 14.04 "Trusty Tahr" - Alpha i386 (20140225)
SourcePackage: openbox
UpgradeStatus: No upgrade log present (probably fresh install)