how do you plug your screen?
s-video, composite, vga, hdmi dvi displayport etc etc?
which videocard do you have?
which os?
i work on nvidia m8600gt on windows seven
to me it happen sometimes only in s-video 2nd output, if i use as first screen a vga monitor and i switch off the laptop screen..
so the s-video become 3rd screen and vga is second, first is off (as windows call them)
but the vjing softwares don't recognise this names and think that 3rd is first, so the output go to vga and preview to the projectors...
i solve this problem, opening the nvidia controlla panel, then connect cables and then select which monitor is on which is off etc etc