their is no XFCE 12 version
you either are using Main(Gnome) version and installed over it XFCE packages
OR usining LMDE (Debian) XFCE. Unless installed previos version (XFCE 10 was ubuntu based I Believein testing no release tho)last release 9
But to your problem...if your using a external monitor from a Laptop the laptop display may be blocking/covering the external monitor from displaying, you must disable laptop display (or setup a dual monitor setup

) in menu-settings-display. If not I would venture the install went bad.
P.S. thats how mine acts (external display on a docked laptop) if I use the laptop it will return to deffault/correct display, so I have to reset the display option when re-docked.(rarely use as laptop anyways) or fiddle with dual display mode!
J.Jay