Dual monitors will not work simultaniously with HP 6550b ProBook?

I have a HP ProBook 6550b notebook-does anyone know why I can’t get the DisplayPort (using DisplayPort to VGA adapter) and VGA external monitor to work at the same time as the built-in monitor of the laptop? My goal is to have 3 screens but I just can’t seem to pull it off. Any feedback/suggestions would be helpful.