indicator notification sometimes not working

I don't know what is the cause of this issue, but I have an intermittent problem with the multifunction keys to adjust the screen brightness (Fn+F5=down, Fn+F6=up). The keys work when "xbacklight" is working, but the notification indicator doesn't show up on the screen. On the other hand, when "xbacklight" is not working, the notification indicator shows up. When xbacklight is not working, I have to reboot and can only hope it would be working again. What is the cause of this and how can I fix this ?

I'm on Lubuntu 16.04 with latest update, nvidia 340.96 driver, "EnableBrightness=1" is set in /usr/share/X11/xorg.d/xorg.conf, xbacklight installed with intermittent functionality (need a couple of reboots), "acpi_osi=Linux acpi_backlight=video" set in /etc/default/grub, "XF86" keys are bound in ~/.config/openbox/lubuntu-rc.xml as following:

    <keybind key="XF86MonBrightnessDown">
      <action name="Execute">
        <command>xbacklight -10</command>
        <startupnotify>
          <enabled>yes</enabled>
          <name>Decrease screen brightness</name>
        </startupnotify>
      </action>
    </keybind>
    <keybind key="XF86MonBrightnessUp">
      <action name="Execute">
        <command>xbacklight +10</command>
        <startupnotify>
          <enabled>yes</enabled>
          <name>Increase screen brightness</name>
        </startupnotify>
      </action>
    </keybind>

Please help !!!

Isao