This may be a timing issue. Apple's Multiple Scan monitors, and some other monitors, are designed with an energy saving feature. When no video signal is being sent to the monitor from the computer, the monitor will go into sleep mode to conserve energy. The PowerBook computer is designed to check if an external monitor is attached during start up, if the computer does not see the monitor it will not send out video signals to save energy.
If you turn on the monitor and let it go into sleep mode and then turn on the PowerBook computer you will get no image on the external display. Likewise, if you turn on the PowerBook first, then turn on the display after the PowerBook has already started up you will not get an image on the external monitor.
The proper way to power up is to turn on the monitor first and immediately turn on the PowerBook. The monitor will be on and awake when the PowerBook is starting up, so the PowerBook will recognize the monitor and send out video signals to the external monitor.