If I remember correctly you need to have the second display on and hooked up before booting the laptop. I have seen that in here several times, though I don't use a laptop so have not experienced it.
I have however forgotten to turn on my mixer which switches my video and pulled my hair out trying to figure out why no display.
And have even had a bad cable because it got crushed too many times under a table leg, so you might try a different cable.