I know this is quite old, but in case you haven't figured this out yet...
Is this an optimus enabled machine, and if so, are you using the Nvidia drivers?
If so, you should change the monitor settings using the nvidia-settings utility - you can launch it from the command line if you can't find it in the GNOME / Xubuntu menus.
If not, you should be able to use the in-built GNOME Displays utility to change your settings in GNOME - I don't know what Xubuntu provides here. If that doesn't work, have a play with the xrandr command from the command line - it will give you feedback as to what's going wrong so you can hunt down the problem.
You can get more info on how xrandr works by running "man xrandr" from the command line, or google it.
(careful switching your primary screen off as you won't be able to see what you're doing!
- I run these in scripts, so it works)
Switching to external monitors:
xrandr --output LVDS1 --off # this switches off inbuilt display first.
xrandr --output HDMI2 --mode 1920x1080 --pos 0x0 --primary --output VGA1 --mode 1920x1080 --right-of HDMI2 #this activates the two external monitors
Switch back to inbuilt display:
xrandr --output VGA1 --off --output HDMI2 --off # switches off the external monitors
xrandr --output LVDS1 --auto #this activates the inbuilt screen again
It's worth pointing out that LVDS1 is my inbuilt display and HDMI2 is my DisplayPort port (no idea why the labelling is off like that).