That's rather odd. I wonder if it isn't trying to use the laptop monitor.
Here's what I would do. i would disconnect the ribbon cable attaching your laptop's screen to the motherboard. I can't say for sure about your laptop but usually it is located beneath the keyboard so ... remove the keyboard ... detach the video cable ... reinstall the keyboard. This should 'force' the laptop to use the external monitor. Do you have or can you get a service manual for your laptop?